I don't feel strongly on a lot of the Pokemon but my Top 5 is probably:
1. Landorus-T
2. Heatran
3. Clefable
4. Dragapult
5. Melmetal
Lando is an auto-include on anything less fat than a very longevity focused Balance or Stall, you know what it does, The Master of Utility- generates tons progress for BO/HO any time it switches in, great stopgap against threats, even uninvested EQ can dissuade opposing Pokemon from setting up in its face (let alone U-Turn into anything else). Outside of the fact that I feel it can struggle with longevity, which isn't a problem for 99% of the teams it finds itself on, it truly is a pretty perfect Pokemon for the format.
Heatran is IMO one of the best breakers in the tier while giving a lot of defensive value, I'd argue Gastrodon is one of the only good splashable defensive checks that doesn't have to run Shed Shell. If you don't have a Gastrodon you just forfeit half your HP to Fire Spin + Magma itself, -> Protect. It adds more 50/50s to a Tapu Lele, a good middleground check to a lot of answers, decent variety in sets. This Pokemon will be a stopgap like Lando-T for tons of the tier, it can't last forever but the amount of progress it gets from correct switches can be crazy at times. Great all-arounder that will break a lot of fat mons, stop a lot of strong mons, and just all-around put in work in every single game.
Clefable is hard for me to rank because I feel like a lot of its different sets are straight up different Pokemon, and that's a bit of the magic, seeing Clefable on preview actually doesn't always tell you what its capabilities are, and it's one of the few defensive Pokemon in the tier that requires scouting. You don't know if it has Knock, Rocks, T-Wave, Trick, CM set, even Life Orb mage until you're letting it in and clicking moves, but that's also because the core traits of just its stats + Moonblast + Soft-Boiled is already such a good check to important threats. When Crown Tundra started, Clefable I felt struggled a bit from the influx of power, but clearly I was wrong back then. It still finds ways to clutch up games that I don't expect it to, Magic Guard can still make it feel unkillable (especially against Balance teams), and it's still going to fit a role on most teams you use.
Dragapult is one of the two broken I would consider "maybe broken" but not actually, it's definitely bullshit at times especially with its SpDef drops, but it's also IMO fine to check, be it hazards, priority, an opposing setup sweeper, or Pokemon that can just take a hit or two -> Kill it back. Its frailness in conjunction with hazards means that while it is 100% a kinda insane momentum and chip generator, which also happens to push itself towards endgame at the end as well, it still needs to be pivoted in by other Pokemon half the time, and I don't see many Dragapult endgames where it's not well setup. One of the most interesting parts of Special Dragapult to me is its fourth move, and I feel that can be surprisingly great at filling in whatever weakness your team has. Hydro Pump is an option of course, hitting Lando/Heatran better as they are both (at least at one point) common SpDef stopgaps, but Thunder is IMO a great counterteam option against Rain, and Flamethrower/Fire Blast are nice as well for Ferro. Overall Dragapult is another Pokemon that puts in work almost every game.
Melmetal is a Pokemon that I feel does so much work even when it's not actually doing anything. Melmetal is a trusty friend on your team that you can switch into on any offensive threat to say "Not today", and in that respect it's also Another Stopgap Check to a lot of threats, but it's also good at making progress itself of course. The reason I put it at the bottom of these 5 is because using a contact move twice in a row in SWSH is a bit tough. That isn't something unique to Melmetal, of course- Urshifu, Weavile, etc. but in the case of Weavile even resisted Triple Axel is doing unga bunga damage, Surging Strikes can make huge progress which makes up the risk... Melmetal kinda just can't click it on the defensive set because it's not worth it. That's where Toxic comes in, but I don't know, maybe I'm just not good at using Melmetal yet (when SWSH was CGOU I kinda stopped playing in 2022, where Melmetal became even more common- I've been using it more nowadays), but I feel like it's just more annoying to make progress with than the rest of these Pokemon. Lando-T can Knock Off things that resist it, Heatran's Magma Storm doesn't give a fuck about resistances, Clefable can run a bajillion moves to shit on you, Dragapult can U-Turn out or SpDef drop fish. Melmetal kinda just has to Toxic things it doesn't like, Earthquake Heatran if it tries to switch in, or generally just accept taking the damage from the attack + hazards and :salute: out.