Honestly, Shaymin should be at least B-. While it gets kinda overshadowed by Celebi it has good all around base stats, the BEST offensive grass type move in the game (Seed Flare) and solid coverage make it a solid offensive threat. HP rock patches up coverage issues allowing it to 1hko Talonflame and Charizard (Pre mega)and pinsir
252 SpA Life Orb Shaymin Hidden Power Rock vs. 4 HP / 0 SpD Talonflame: 385-458 (129.1 - 153.6%) -- guaranteed OHKO
252 SpA Life Orb Shaymin Hidden Power Rock vs. 4 HP / 0 SpD Charizard: 328-390 (110 - 130.8%) -- guaranteed OHKO
52 SpA Life Orb Shaymin Hidden Power Rock vs. 4 HP / 0 SpD Thundurus: 172-203 (57.3 - 67.6%) -- guaranteed 2HKO
252 SpA Life Orb Thundurus Hidden Power Ice vs. 0 HP / 4 SpD Shaymin: 166-198 (48.6 - 58%) -- 96.1% chance to 2HKO
Thundurus can't switch in on an HP rock
252 SpA Life Orb Shaymin Hidden Power Rock vs. 4 HP / 0 SpD Mega Pinsir: 312-369 (114.7 - 135.6%) -- guaranteed OHKO
252 SpA Life Orb Shaymin Hidden Power Rock vs. 4 HP / 0 SpD Pinsir: 190-226 (69.8 - 83%) -- guaranteed 2HKO
252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 0 Def Shaymin: 194-230 (56.8 - 67.4%) -- guaranteed 2HKO
Pinsir can't switch in after an HP Rock and is OHKO'd if it has mega evo'd
Also some other relevant calcs:
252 SpA Life Orb Shaymin Seed Flare vs. 252 HP / 4 SpD Assault Vest Conkeldurr: 200-238 (48.3 - 57.4%) -- 93.8% chance to 2HKO
252 SpA Life Orb Shaymin Earth Power vs. 252 HP / 4 SpD Aegislash-Shield: 151-179 (46.6 - 55.2%) -- 12.9% chance to 2HKO after Leftovers recovery
252+ SpA Aegislash-Blade Shadow Ball vs. 0 HP / 4 SpD Shaymin: 160-189 (46.9 - 55.4%) -- 69.5% chance to 2HKO
Aegislash can't stay in versus Shaymin nor switch in
Also note that Timid was used in all of these calcs.
I know this isn't that many threats but the point is is that offensive LO Shaymincan get past some of the top metagame threats with good prediction. The downsides are mono grass typing and requiring prediction to KO things like Talonflame. Otherwise Shaymin is really solid. Plus it's super cute!
I love Shaymin, as it's one of my favorite Pokemon. However, I think C- is probably the best rank for it. Firstly, Celebi gives it serious competition. Not only does it have the ever-reliable recovery move in Recover, but it also has access to Ancient Power. While it may not seem like a big deal, Ancient Power allows Celebi to have 31 Speed IVs, unlike Shaymin, who must run 30 Speed IV's in order to use Hidden Power Rock. This means that Celebi can achieve Speed ties with Pokemon who have a base 100 Speed, most notably Charizard Y, who is OHKO'd after a little prior damage(252 SpA Life Orb Celebi Ancient Power vs. 4 HP / 0 SpD Mega Charizard Y: 255-302 (85.5 - 101.3%) -- 6.3% chance to OHKO) Celebi also brings diversity, meaning that when it's sent out, you won't know what set it's running, what moves it has, it's item, etc. Shaymin, on the other hand, will almost always be running Seed Flare/Earth Power/Hidden Power Fire/Rock/Recovery move, meaning it's extremely predictable.
Roserade is another Grass-type who gives it competition. Roserade can support the team very well with Spikes, Sleep Powder, and Toxic Spikes, while also hitting decently hard with a base 125 SpA. It also has a nice STAB Sludge Bomb to work with.
Shaymin's biggest competitor, however, is Mega Venusaur. This thing has great bulk, but it also has good 122 SpA and a 100 Atk to work with. Combine this with moves such as Giga Drain, Earthquake, and Sludge Bomb, and Venusaur can be a great tank. It does all this while also having access to a decent Recovery move, making it one of the best Special Attaking Grass-types in the tier.
Overall, Shaymin has it's niche of firing off STAB Seed Flares, but it faces a lot of competition that
leaves it at C-
Edit: Yeah, I also support
Deo-S for S rank. This thing is so diverse, being able to run hazard lead/Dual Screens/Weather setter all while having a massive 180 Speed that allows it to use these moves quickly and effectively. It's revenge killing set is also amazing when used correctly, as a lot of set-up sweepers/fast Pokemon can be taken out quickly by it's large movepool and Life Orb boosted attacks. All in all, I definitely support S-rank for Deo-S.