I know I said "we don't bite", but you should have at least an understanding of how these things (VR threads) minimally work because your post shows a lacking in this regard.
Ah yes, the classic
appeal to authority response. While I appreciate your work in maintaining this list, has it occured to you that a significant portion of people responding to this thread may be basing their opinions off of their standard OU experience or theorycrafting that has very little application in the actual metagame? Believe it or not I do know how these kinds of threads work, and I also know that Pokemon have been radically shifted up and down all sorts of viability tier lists as the metagame has shifted or become better understood. Just look at what happened to Mega Pinsir or Quagsire in OU- opinions change. The way I see it, we can either look at
battle after
battle after
battle after
battle after
battle after
battle after
battle after
battle after
battle and ask "hey, I wonder why not a SINGLE TOP PLAYER is using Landorus", or we can put our heads in the sand, shut down any debate on the subject, and wonder why 98% of the people on the ladder who probabaly come to threads like this for advice have no clue what they're doing.
So my question to you is, do you want have a serious debate about this or not?
This nomination makes no sense and I will sum up why as the bolded parts are wrong. Sure it's overprepared for but if anything that shows how much of a dominating presence in the meta and the massive strain it puts on teambuilding. Azumarill loses to sludge wave while conkeldurr hates psychic but even then earth power hurts them a lot. Also landorus is not scary is false as this thing's presence basically makes you really hesitant to switch in. While weavile and mammo can revenge it they can't switch in. Plus spdef gliscor loses to hp ice ones and cresselia only fits in stall.
Kyurem-B can beat every single one of its counters with the right coverage moves- that alone does not make a Pokemon S-tier. Forcing switches is nice on paper, but you have to ask yourself how often Landorus is going to be forcing switches in the first place. It needs to be able to comfortably beat most common threats 1v1 before it can even start to think about how much damage it can do on the switchin. This is especially important because Landorus does not have the best speed or bulk and sports two easily exploited weaknesses. My argument is that the extra team space afforded by the lack of Megas makes it easier to use Pokemon that deny Landorus these opportunities. The very common Assault Vest Azumarill and Conkeldurr sets both force Landorus out, as they can easily stomach a super effective hit and OHKO back with Waterfall/Ice Punch:
252 SpA Life Orb Sheer Force Landorus Sludge Wave vs. 240 HP / 16 SpD Assault Vest Azumarill: 255-302 (63.5 - 75.3%) -- guaranteed 2HKO
252 SpA Life Orb Sheer Force Landorus Psychic vs. 0 HP / 236 SpD Assault Vest Conkeldurr: 213-252 (60.6 - 71.7%) -- guaranteed 2HKO
Even the Choice Band Azumarill set has a good chance to OHKO Landorus with Aqua Jet after Stealth Rock, which Landorus has little to no chance of preventing. As for other big OU threats, we can just go down the list and see how well each of them do in a 1v1 matchup scenario:
Azumarill- Lando loses to vest/banded sets, takes severe damage from others
Bisharp- Lando wins, but takes ~50% damage from Sucker Punch
Keldeo- Lando always loses
Clefable- Lando wins if it has Sludge Wave but cannot OHKO usually, loses to Ice Beam sets
Garchomp- Offensive sets can outspeed and ohko Lando with Outrage, defensive sets can tank HP Ice and get up Stealth Rock
Gengar- Lando loses to Icy Wind, takes severe damage from Shadow Ball
Gliscor- Can tank an HP Ice and force Lando to take 2 turns of toxic damage, can toxic stall out sets lacking the move
Heatran- Scarf set outspeeds and OHKOs with Overheat, slower sets forced out
Landorus-T- Lando with HP Ice wins
Latios- Lando loses to Draco Meteor
Talonflame- Lando loses to Brave Bird
Thundurus- Lando loses to HP Ice
And these are just the S and A+ viability mons... going down to A you have Latias, Raikou, Manaphy, Gyarados, Mamoswine, Slowbro, Rotom-W, Scizor, Starmie, and Tornadus-T who can all reliably beat Landorus's usual sets. There's a pretty consistent pattern going on here- Landorus tends to straight up lose to half the Pokemon in the tier, has iffy matchups against another quarter of them, and is either crippled or takes a significant chunk of damage from the Pokemon it does force out if they decide to stay in. Considering that its main job is to be a wallbreaker and offensive cleaner, it really gets hugely outclassed by other offensive threats like Latios and Keldeo. Landorus is good, don't get me wrong, but every piece of evidence both theoretical and from actual battles suggests that it's being tremendously overrated.
As for Cresselia, well, being able to switch into a lot of revenge killers and spread around Paralysis is extremely useful for offensive teams. It has great bulk on both sides, something typical stall Pokemon like Chansey and Skarmory do not, meaning you can cover a huge amount of threats in one teamslot while relaxing your restrictions on what kind of offensive threats the rest of your team will use. From these reasons and my experience I've found it extremely useful for offensive teams as well as stall.