Punchshroom
FISHIOUS REND MEGA SHARPEDO
Gonna have to talk about this one. Balance is nice, but specialization is arguably more important in the OU metagame. Stuff like Breloom, Volcarona, and Terrakion may be somewhat predictable, but it doesn't necessarily make them less efficient in their roles. Mew may be able to perform many roles but it doesn't excel in one particularly well enough to be considered over another potential user. I believe this very lack of specialization is the main reason Virizion is sitting pretty in UU despite it being great against Rain.Well, this is the problem.
Everybody says it is outclassed by something or the other, because "there are better sweepers in each category".
But with a lot of those sweepers, your opponent has a fair idea of what you might be running(for instance, Terrakion is almost guaranteed to have Close Combat and Stone Edge) .
Jack-of-all-trades, Master of none is a cliche. In reality, balance across everything is key to success. An imbalanced pokemon with VERY high attack but bad other stats like Rampardos for instance doesn't do well at all. Similarly, Shuckle may have some of the best defenses in the game but with a lack of balance, it can't do well at all. Mew has balance. Agreed, there are better SD sweepers. Agreed, there are better Nasty Plot sweepers. But which mon apart from Mew can claim to have access to both, meaning your opponent has no clue what to actually use against it. Special Walls fear the SD set, while vice versa for physical walls. You can't even status Mew because you get statused as well and Mew can always Heal Bell itself. Mew can actually do well on many teams because it can give SO much coverage that your opponent has no idea what he can ACTUALLY switch into it.
Well, I've been nominating it for A-, because despite everybody calling it a jack-of-trades, it is a serious threat to sweepers who have no idea what to hit it with, making it a GREAT late-game cleaner
Jirachi is Mew's biggest competitor in the supporting role. While Mew has better recovery, Jirachi has much more resists in Dragon, Ice, Rock....and less weaknesses, one of which is reduced in Rain, all of which are important advantages over Mew. To top it off Jirachi can Wish pass as well. Celebi gives Mew a run for its money as well, sharing Nasty Plot, Baton Pass, Recover and status deterent (virtual immunity for that matter), while packing excellent resists in Water, Electric and Ground.
Offensive Mew may be able to pull off a sweep due to surprise value, but keep in mind that since people have been arguing that Alakazam's STAB isn't strong enough to finish numerous targets, consider that Mew's average 100 base Sp.Atk means it pretty much has to boost so that it can hit hard enough, and doesn't get cool STAB Giga Drain like Celebi does. Physical sets are even worse off due its STAB being even weaker and inaccurate, just that it has cooler coverage moves in priority Sucker Punch and healing Drain Punch, but this isn't really what you're looking for in Mew.
This may or may not come as a surprise to you, but Mew's biggest crutch is its typing. Think about it, weaknesses to Bug and Dark are painful due to Scizor and Tyranitar's omnipresence (the only relevant Ghost attacker is Gengar who doesn't OHKO, but does outspeed), while it resists just Fighting and Psychic, which essentially just translates to just one resist since Psychic offense is hardly relevant in a metagame where 2 of the best Pursuit trappers reside. The main Fighting types in Keldeo and Terrakion can lay the beat down on offensive Mews with their other STAB while outspeeding it, meaning a direct switch-in isn't really safe, while defensive Mews compete with the likes of Jellicent, Celebi or even Reuniclus to combat these threats.
Mew may be a jack of all trades, but the "master of none" part is really prominent when it faces a lot of competition in OU. Top or Mid B Rank from me.