Someone mind telling me how "Diancie is the objectively best fairy in the tier"? (i assume you mean offensive fairys since its certainly not better than the master of yellow magic himself right?)
I've read the this notion ever now and then through out the last year and i never really understood it. In terms of power Diancie is pathetic compared to Gardevoir resulting in the fact that pretty much every mon with halfway decent mixed bulk that isnt hit SE can easily wall it, starting with standard Clef over Hippo (not the standard smogon spread but its easy to adjust without much sacrifice), Mew, Cele, Slowking, AV Tangrowth, Amoonguss, Venusaur and pretty much every steel type in the tier (Tran/Ferro only depending on Diancies coverage). Its just so much easier to switch in to compared to Garde, especially considering that the likes of Skarm and Chansey dont have to worry about getting taunted and hence beeing screwed the next time they try to come in. Because of that its usually deadweight against stall and most balance teams, against offense the higher speed is nice sure, but about half the mons you will find on your average offense team are still faster and ohko or at least tie with it. Then there is this annoying fact that you have to carry protect around to even mega evolve safely limiting your coverage further and killing your momentum in many cases. I guess it can check a bunch of things like torn, talon, bisharp, keld etc but to be "the best fairy in the tier" there should be more to it i think?
I still think
thunder wave clefable is objectively (offensively and defensively) the best fairy in the tier, and I realize we probably agree on that point, but boy are you downplaying diancie lmao.
First off, there's this really big misconception that mega gardevoir is some kind of god-like nuke. In reality, it's just strong enough to 2hko bulky shit, making it a solid wallbreaker with good fairy STAB. It doesn't OHKO shit like mega medi and char-y do (the real god-like nukes) with no sweat. But let's show a comparison of damage for now:
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. 252 HP / 224+ SpD Mew: 166-196 (41 - 48.5%) -- guaranteed 3HKO after Leftovers recovery
224 SpA Mega Diancie Moonblast vs. 252 HP / 224+ SpD Mew: 132-156 (32.6 - 38.6%) -- 2.6% chance to 3HKO after Leftovers recovery
(bad mew spread, ik, but that's what calc had+too lazy to change it)
Yeah, you're right, mega gardevoir's way stronger, but as far as mixed coverage goes, a psyshock from garde is quite a bit weaker than diancie's diamond storm:
32 Atk Mega Diancie Diamond Storm vs. 252 HP / 172 Def Clefable: 174-205 (44.1 - 52%) -- guaranteed 3HKO after Leftovers recovery
232 SpA Mega Gardevoir Psyshock vs. 252 HP / 172 Def Clefable: 162-192 (41.1 - 48.7%) -- guaranteed 3HKO after Leftovers recovery
It's funny that you mention mons such as mew, celebi, and slowking though, all of them can run enough sdef to quite comfortably avoid the 2hko from hyper voice, necessitating taunt to break them. Against things like chansey and skarm, taunt itself is a risky move when they can stoss or iron head, or if you attack predicting them to predict taunt, and if they predict that the could just roost/softboiled off the damage or set hazards/t-wave or toxic you. Stoss might not seem like much, but when garde has <300 hp and fails to even 3hko 100% of the time with psyshock, it has significant implications, especially when garde is your biggest player vs stall.
I dislike how you're looking at the stall matchup in a vacuum too. Implying that mega diancie is useless because every mon is at full health is just not how a stall matchup works. No wallbreaker is totally independent (and any that are we've banned lol see hoopa), and no wallbreaker would realistically be tasked with single-handedly dismantling such a potent playstyle. Both mega gardevoir and mega diancie need partners to chip and pressure shit like chansey, amoonguss, and skarmory, and when that happens, they can actually do shit. Mega gardevoir is stronger against stall than mega diancie that's for sure, but their damage output isn't that different.
What IS different is that mega diancie has a fucking 110 base speed, one of the biggest benchmarks in the tier, which is freaking fantastic. As you ever so conveniently stuffed in the last sentence yourself, mega diancie checks/revenge kills several key threats in the metagame like torn, talon, bisharp, keld, pinsir, mega latias, char-x/y, and garchomp; many of which are very difficult for offense to handle easily. Having an offensive check to all of them that can still maintain offensive pressure while still being able to pressure bulky shit with some offensive support is the reason mega diancie is solidly A+ and that mega garde is being nommed down to A-
I also think you're underestimating protect's utility. It makes your matchup vs mega lopunny far better, as the opponent must now play around protect to kill with hjk instead of outright revenge killing. Protect can scout the moves on shit like scarf rachi, scarf lando-t, scarf keldeo, scarf chomp (rarer), and even iron head scarf kyu-b (very rare lol), making it a little difficult to revenge kill as well. Limiting coverage is fair enough, mega diancie would appreciate epower, psyshock, rock polish, calm mind and all that jazz, but protect doesn't always kill your momentum. You say it kills momentum in "many cases," but in just as many cases it can gain you momentum as well.
For example, if you have a garchomp vs diancie turn 1 matchup and your opp has a mega scizor in the back, you could just mega evolve and hp fire turn one, getting you tons of momentum and a crucial kill early on. The presence of protect can both take and give momentum, and that all boils down to prediction. It's unfair and inaccurate to dismiss that it kills your momentum and leave it at that.