While I agree Gallade can be a big nuisance, I believe there's a Pokemon out there that's a sleeper threat, Flygon. Let the haha's roll in.
This Pokemon just has so much versatility and that is what makes it a pain to take care of. Obviously, there are the bulky Stealth Rock sets which aren't scary, but the rest (Choice Band, Offensive Stealth Rock, Eject Pack, Dragon Dance, Choice Scarf) is where lines blur and checks become less common between them.
Choice Band Flygon in the current metagame is a very powerful wallbreaker that can easily beat down Bronzong and its Grass-type checks with a powerful U-turn, which it can click repeatedly. Your best bet to check it defensively is fish for burns with Effect Spore, Scald, and Flame Body or use Orthworm. Now, don't get me wrong Orthworm is a good Pokemon, but it's not exactly slappable on too many teams. And, for the other checks, Flygon 2HKOes both Vileplume and almost always Vaporeon (98.4%) after Stealth Rock. Flygon could also just use U-turn on the latter before getting hit, chipping it and bringing in a threat. I've also seen Flygon recently go for Rock-type attacks (Stone Edge for power of Rock Slide for reliability) to OHKO Talonflame upon switch-in. Some have even chosen Outrage to just have an easy nuke button versus fatter teams. Counterplay also exists offensively, but thanks to its not pitiful bulk and gen 3 stats, it tends to take an attack or two from even offensive Pokemon if needed. Tera Steel has also caught on more recently to let it more easily live attacks so that it can continually terrorize your opponent. And, for those faster Pokemon, First Impression still hurts and can easily pick off threats.
Offensive Stealth Rock, while being the least threatening set, is still an important one to note nonetheless. Normally, this set carries the utility of First Impression, which is great on balance builds to check hyper offense or faster Pokemon in general, but this means Talonflame can come in easily and use Defog to clear the Stealth Rock. This has lead me to run Rock Slide over First Impression at times, as it can OHKO Talonflame upon switch-in or on a Defog and alongside Earthquake, it can cause uncomfortable 50/50s for them if they've clicked Will-O-Wisp instead. Finishing off the set, it also has access to U-turn, ensuring it isn't passive and helps to keep up momentum.
Eject Pack Flygon is an interesting set that people aren't using enough. Flygon easily calls in physical walls like Vileplume, Talonflame, Swampert, and Tsareena, which all come in to get Stealth Rock set up on them and then are hard by a Draco Meteor while Flygon keeps up momentum by switching to a threatening partner. Flygon could even choose to stay in and get some chip on checks like Bronzong and Orthworm with Fire Blast. The most dangerous part of this set is the surprise factor and how it can weaken physically defensive Pokemon for its partners. Even after it's clear that Flygon is a special attacker, very little can handle its STABs + Fire Blast comfortably. It can be a slight momentum sink if Draco Meteor misses or if Eject Pack already got used up, but the former is just unfortunate Pokemon luck and the latter would only happen if it threatens the opponent currently, which can take advantage with double switches or by just getting some chip. Also, Eject Pack sets have also been found to run Earthquake with a Hasty Nature for a stronger STAB move to hit the non-Ground immune Steel-types with, as all of those are specially defensive.
Dragon Dance Flygon is the most threatening setup sweeper to me. Its already decent Speed tier becomes hard to handle after it's doubled with Dragon Dance + Scale Shot, which it will almost always get off. And, with Substitute becoming more popular to avoid status ailments and to take advantage of weaker Pokemon, Flygon is a Pokemon that can easily spiral out of control if you don't react correctly and quickly enough. Substitute + Tera Steel is especially annoying to handle, as in most cases you need to switch out because of how Flygon uses what would've normally been your check, like Vileplume, Tsareena, or Talonflame, as set up fodder now. There's also been some small development with Dragon Dance sets now containing an EV spread of 32 HP / 224 Atk / 252 Spe to ensure their Tera Steel Substitute doesn't get broken by Talonflame's Brave Bird. And, while more uncommon, Tera Fire + Fire Punch sets still exist to hit Bronzong, Orthworm, and Magnet Rise Klefki which could be a decent countermeasures otherwise.
Choice Scarf Flygon is the most slappable set and the most popular because of it. Its great Speed tier, utility, typing, and ability make it a strong addition to all teams because it helps offensively and defensively. Stealth Rock and U-turn lets you capitalize on switches and Outrage can provide an easy way to clean games. This is walled decently by Talonflame however, so if you can fit Stealth Rock elsewhere, it's worth it to have Stone Edge or Rock Slide to OHKO Talonflame as it tries to check you. With Levitate also ignoring both Sticky Web and Spikes and its resistance to Stealth Rock, Flygon makes for the premier Choice Scarf user for Boots Spam and regular balance teams alike. It is outsped by other common Choice Scarf users and walls can easily handle it with how it can be locked in to a fairly weak move, but with it being a more supportive Pokemon, this isn't that big a deal. Its checks offensively change, however, as it now beats all non-Choice Scarf users and even handle some boosted threats like Oricorio-Pom-Pom and Torterra. Tera Steel is nice to survive a hit from these threats if needed, and Tera Ghost has risen a bit to better handle Lucario's Extreme Speed, blanking it completely.
With how all Flygon sets have different checks and with certain sets being able to beat other sets' checks, I believe Flygon is starting to become a bit much for the tier. While it isn't the absolute worst thing right now because we have other things to deal with (possibly Gallade, Lucario, Cloyster, Cresselia, [setup sweeper #5], etc.), I believe we should all keep an eye on this Pokemon, as it could easily be banned once all the other strong setup sweepers have been dealt with (whether that be through ban or meta evolution). This isn't exactly a surprisingly new take either, as we've already seen some support from council members in the past two votes from Pokeslice, Dr. Phd. BJ, and zS.
This Pokemon just has so much versatility and that is what makes it a pain to take care of. Obviously, there are the bulky Stealth Rock sets which aren't scary, but the rest (Choice Band, Offensive Stealth Rock, Eject Pack, Dragon Dance, Choice Scarf) is where lines blur and checks become less common between them.Choice Band Flygon in the current metagame is a very powerful wallbreaker that can easily beat down Bronzong and its Grass-type checks with a powerful U-turn, which it can click repeatedly. Your best bet to check it defensively is fish for burns with Effect Spore, Scald, and Flame Body or use Orthworm. Now, don't get me wrong Orthworm is a good Pokemon, but it's not exactly slappable on too many teams. And, for the other checks, Flygon 2HKOes both Vileplume and almost always Vaporeon (98.4%) after Stealth Rock. Flygon could also just use U-turn on the latter before getting hit, chipping it and bringing in a threat. I've also seen Flygon recently go for Rock-type attacks (Stone Edge for power of Rock Slide for reliability) to OHKO Talonflame upon switch-in. Some have even chosen Outrage to just have an easy nuke button versus fatter teams. Counterplay also exists offensively, but thanks to its not pitiful bulk and gen 3 stats, it tends to take an attack or two from even offensive Pokemon if needed. Tera Steel has also caught on more recently to let it more easily live attacks so that it can continually terrorize your opponent. And, for those faster Pokemon, First Impression still hurts and can easily pick off threats.
Offensive Stealth Rock, while being the least threatening set, is still an important one to note nonetheless. Normally, this set carries the utility of First Impression, which is great on balance builds to check hyper offense or faster Pokemon in general, but this means Talonflame can come in easily and use Defog to clear the Stealth Rock. This has lead me to run Rock Slide over First Impression at times, as it can OHKO Talonflame upon switch-in or on a Defog and alongside Earthquake, it can cause uncomfortable 50/50s for them if they've clicked Will-O-Wisp instead. Finishing off the set, it also has access to U-turn, ensuring it isn't passive and helps to keep up momentum.
Eject Pack Flygon is an interesting set that people aren't using enough. Flygon easily calls in physical walls like Vileplume, Talonflame, Swampert, and Tsareena, which all come in to get Stealth Rock set up on them and then are hard by a Draco Meteor while Flygon keeps up momentum by switching to a threatening partner. Flygon could even choose to stay in and get some chip on checks like Bronzong and Orthworm with Fire Blast. The most dangerous part of this set is the surprise factor and how it can weaken physically defensive Pokemon for its partners. Even after it's clear that Flygon is a special attacker, very little can handle its STABs + Fire Blast comfortably. It can be a slight momentum sink if Draco Meteor misses or if Eject Pack already got used up, but the former is just unfortunate Pokemon luck and the latter would only happen if it threatens the opponent currently, which can take advantage with double switches or by just getting some chip. Also, Eject Pack sets have also been found to run Earthquake with a Hasty Nature for a stronger STAB move to hit the non-Ground immune Steel-types with, as all of those are specially defensive.
Dragon Dance Flygon is the most threatening setup sweeper to me. Its already decent Speed tier becomes hard to handle after it's doubled with Dragon Dance + Scale Shot, which it will almost always get off. And, with Substitute becoming more popular to avoid status ailments and to take advantage of weaker Pokemon, Flygon is a Pokemon that can easily spiral out of control if you don't react correctly and quickly enough. Substitute + Tera Steel is especially annoying to handle, as in most cases you need to switch out because of how Flygon uses what would've normally been your check, like Vileplume, Tsareena, or Talonflame, as set up fodder now. There's also been some small development with Dragon Dance sets now containing an EV spread of 32 HP / 224 Atk / 252 Spe to ensure their Tera Steel Substitute doesn't get broken by Talonflame's Brave Bird. And, while more uncommon, Tera Fire + Fire Punch sets still exist to hit Bronzong, Orthworm, and Magnet Rise Klefki which could be a decent countermeasures otherwise.
Choice Scarf Flygon is the most slappable set and the most popular because of it. Its great Speed tier, utility, typing, and ability make it a strong addition to all teams because it helps offensively and defensively. Stealth Rock and U-turn lets you capitalize on switches and Outrage can provide an easy way to clean games. This is walled decently by Talonflame however, so if you can fit Stealth Rock elsewhere, it's worth it to have Stone Edge or Rock Slide to OHKO Talonflame as it tries to check you. With Levitate also ignoring both Sticky Web and Spikes and its resistance to Stealth Rock, Flygon makes for the premier Choice Scarf user for Boots Spam and regular balance teams alike. It is outsped by other common Choice Scarf users and walls can easily handle it with how it can be locked in to a fairly weak move, but with it being a more supportive Pokemon, this isn't that big a deal. Its checks offensively change, however, as it now beats all non-Choice Scarf users and even handle some boosted threats like Oricorio-Pom-Pom and Torterra. Tera Steel is nice to survive a hit from these threats if needed, and Tera Ghost has risen a bit to better handle Lucario's Extreme Speed, blanking it completely.
With how all Flygon sets have different checks and with certain sets being able to beat other sets' checks, I believe Flygon is starting to become a bit much for the tier. While it isn't the absolute worst thing right now because we have other things to deal with (possibly Gallade, Lucario, Cloyster, Cresselia, [setup sweeper #5], etc.), I believe we should all keep an eye on this Pokemon, as it could easily be banned once all the other strong setup sweepers have been dealt with (whether that be through ban or meta evolution). This isn't exactly a surprisingly new take either, as we've already seen some support from council members in the past two votes from Pokeslice, Dr. Phd. BJ, and zS.
Toxicroak
Vileplume
Lures
Strong Specs Users
Ground Types
Knock Resists (with intimidate!)
Bulky Setup (with tera poison)
and 
can be considered as Stallbreak Slow Set-Up Sweepers.
and 
are emerging as problematic only because they are the last viable StallBreakers left. The ban of 
and 
was probably too much (I almost never used them and never found them problematic).