Can we just stop talking about Mew please? Keldeo333, not to be rude, but you should realize that everyone disagrees with you and move on...
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However, more importantly, I would like to nominate Ferrothorn for A-tier.
Honestly there is probably not any Pokemon in OU that is such a useful utility Pokemon, and it has a pretty nice movepool for a support Pokemon. With Spikes and Stealth Rock within its reach, it automatically is a great support Pokemon in a game where entry hazards are so useful for KOs. However, it has a lot of stuff that makes it at an advantage over Forretress, Skarmory, and other Pokemon in the metagame. Grass/Steel is one of the best typings in the whole game, and while its weak to Fighting, it is capable of dealing with many, many, many Water-types such as Politoed, Gastrodon, and more. Its typing also does other helpful things for it - I've seen many Latias unable to damage Ferrothorn while I did a considerable amount back to it with Gyro Ball and Leech Seed. Speaking of Leech Seed, that move is outsanding, capable of racking up a ton of residual damage on the foe. Most Pokemon hate it a lot. Ferrothorn can also suprise foes with its inability to let them set up. You got a Terrakion? If Ferrothorn can predict a switch and use Gyro Ball, it is not a happy Terrakion to say the least.
The dominance of rain is also very useful for Ferrothorn in many ways. It takes nothing from Politoed, Jolteon is useless against it, it OHKOs Gastrodon easily, and even the occasional Rain Stallers that show up find it difficult to get Ferrothorn out of the way - without Toxicroak or a Therian, Rain teams really struggle against Ferrothorn without a lucky burn from Scald (which isn't unseen for sure). On the defensive end, Ferrothorn is also a very capable Pokemon in the rain, where most Hidden Power Fires do pathetic damage without a boost, and unlike Forretress it has the bulk to take Special hits - 74/131/116 defenses are really good on both sides and the average HP being mitigated by those great defenses helps it to get back up there with Leech Seed easier.
Finally, Ferrothorn is excellent against a Dragon-type Pokemon that is locked into Outrage or using Draco Metoer. Ferrothorn takes on Draco Metoers better than most Steel-types too. Salamences, Garchomps, and Kyurem-Bs locked into Outrage are almost useless against Ferrothorn, and residual damage from Iron Barbs and Leech Seed, in addition to a possible Gyro Ball hit. Heck, I've found Kyurem-B mediocre almost for the sole purpose of Ferrothorn, I kid you not! Latios is checked even with Hidden Power, as a Draco Meteor won't do that much, and a -2 Hidden Power Fire, possibly when combined with Rain, is not doing enough to make Ferrothorn fold but a Gyro Ball will hurt.
While the right moves can take it out easily and it suffers from some competition, Ferrothorn is definently a Pokemon deserving of A-Rank. It has so many useful traits, and there is a big reason why it is used a considerable amount more than other Steel-types.