Shaymin is not outclassed by Celebi lol they aim to do completely different things. Seed Flare is annoying to deal with as well as making Shaymin a potent wall breaker and is the reason it's ranked.
Venusaur is a staple on sun teams and is legitimately threatening with access to Growth. Surprisingly hard to wall with options like Sleep Powder to surprise its checks. Plus it's the best Chlorophyll user so it'll always have a spot in sun teams, unlike Ludicolo who struggles to justify a team slot.
Celebi is an entire 3 letter ranks higher than Shaymin. It has a niche in Seed Flare, and either a help or hindrance for your team with pure Grass typing, but it's outclassed in every other regard from the difference in their movepools. Perhaps in practice the gap between the two may not be so severe, but there is a reason you never see Shaymin around.
Haxorus - OK, C+ is WAY too high of a nomination for it, but C- isn't out of the question, even if I don't think it necessarily deserves it. While the Mega DD users are indeed overwhelmingly better for their bulk and typing. However, people overlook Haxorus' ability to hold an item. In practice, while Gyarados can avoid status with Taunt/Substitute, and Char-X is immune to burn (but not paralysis), Haxorus has the benefit of being able to hold Lum Berry which it can afford to do because of just how crazy strong it already is. Anyone who runs or encounters SD+Lum Garchomp knows just how devastating in practice a free turn to boost is when you get to capitalize on the opponents expectations. This can cost your opponent the match. It's also a good Double Dragon partner as, ironically, it's one of the best things to clear would be checks for other Dragons like Char-X. EQ slaughters Heatran unboosted. Poison Jab maims Azumaril and Clefable, etc. It's basically the bouncer pokemon going "move bitch, get out da way!" for your prominent sweeper.
All that considered, I still wouldn't use DD, SD+Lum is the way to go. It's horrible against offense but it isn't so frail it cant survive unboosted neutral hits, so it can at least just hit Outrage coming in fresh against Mega Manectric for instance and kill or cripple something.
Charizard X - I'm having a hard time nominating for S currently. Like AM said, the environment is pretty hectic at the moment, and frankly, sweeping with Char-X isn't as easy as people are making it out to be. If you can't get rocks out of the way by the time you have to send him in, and you're relying on Flare Blitz to take something out, that is a lot of wasted potential right there. It's hard finding the time to Roost off hazard/passive damage, get your boost of choice off, and then start attacking. In practice it requires a lot more support to play optimally, hell, even get onto the field compared to most of the other A+ rankings. S is really stretching it at this points, as while its the upper crust of A+, those S mons are just so much easier to use. For now
keep Charizard X in A+
Mega Venusaur - Many things have certainly shifted in Venu's favor lately, and a Grass type with no Fire or Ice weakness is as awesome as ever. But I think A+ at the moment is overselling it. It is simply much easier to wear down than people think. While it doesn't take much hurt from Scald, Burn is the death of this thing, and Paralysis is equally annoying. Synthesis is hampered by Sand (which is predicted and showing some comeback) and has woefully low PP. Giga Drain is nice but not
reliable recovery, especially not against resists. You have no Leftovers and you take Sand, rock, and spikes damage. To get the most out of Megasaur you really really want cleric/wish support and hazard removal, as otherwise it isn't too difficult to get him to a level where something faster can just one or two shot him. Offensive sets seem to be doing a bit better, but I don't think Megasaur is on the same level as Lopunny, Scizor, Thundurus, Tornadus-T, Talonflame, Char-X or Bisharp... yeah, that's a lot.
Mega Venusaur stays A
Stunfisk - You know, I really enjoyed those replays earlier. Cockblocking Volt Switch and being able to punish Flying types between Electric or Ground when they Roost is amusing to me, and its nice to have a rock setter than can force switches with Yawn. It's rare to get a setter that can keep control on your side like that. I support
D, I kinda want to try it.
Nidoking/Queen - Does anyone actually have experience with these guys? Theoreywise it makes all the sense for D rank but can people at least post replays like they did for Stunfisk?
Chesnaught - I'm ok with this dropping. The meta is more special oriented than its ever been and Chesnaught isn't cut out for that. Skarmory far more reliably lays Spikes down and Roost and Whirlwind are better than Synthesis and Drain Punch/Leech Seed. It's niche isn't so useful atm, so dropping to
B- is justifiable.
Amoongus - I think its still a cut above Chesnaught. That Poison typing for Fairy types and Regenerate, plus Spore, are all huge advantages it has over Chesnaught, and it need not be so passive when it has access to Foul Play, so things can't safely set up on it even if you already put something else to sleep. It's also a Keldeo check not easily worn down from Scald burns thanks to Regenerator, so it has plenty of uses. It just has to be implemented properly. Should stay
B, or above Chesnaught.
Mandibuzz - I like what somebody said to not run it as a Defog user. Its rare to have something as bulky as Mandibuzz that can be a monkeywrench for both offense AND stall with a simply set of Taunt, Roost, Foul Play and Toxic/Whirlwind. It can stall strong attackers with Roost and Rocky Helmet/Toxic. It's also pretty good at checking Metagross, Alakazam, Scizor, Starmie and if people are saying Reuniclus should raise than Mandibuzz should NOT be dropping when it straight up counters it. People overlook the usefulness of a Dark type not weak to Fighting, and as bulky as Mandibuzz is with recovery. It should stay in
B-, and honestly don't be surprised if it it gets nominated to move up soon based on meta trends.