ah crobat I've lost count of how many times this things pissed me off.
Leftovers Inner Focus Jolly
Moveset EVs
~ Taunt
~ Roost
~ Brave Bird
~ Super Fang / Toxic
252 HP / 4 Def / 252 Spe
With Crobat's blazing Speed and resistance to Fighting-type moves, along with a valuable Ground-type immunity, it can quite effectively come in on Choice-locked attacks, Taunt the opponent, and proceed to cause havoc with a combination of Super Fang and Brave Bird. This set is effective as the majority of defensive Pokemon that try to wall Crobat are crippled by Taunt and Super Fang, while not many offensive Pokemon will like taking a STAB Brave Bird.
Taunt is the main move of the set, and it is practically the only reason to use Crobat in OU. With Taunt and Roost, Crobat can continuously recover in the face of defensive Pokemon while wearing them down with Super Fang and Brave Bird. The remaining moves on the set are to give Crobat some offensive prowess. Brave Bird is used as a general STAB move, which still hits hard despite Crobat's mediocre base 90 Attack stat. Finally, Super Fang is used in the last slot to bring down Pokemon into the KO range of Brave Bird; this is a very effective tactic, as only Ghost-types are immune to Super Fang, most of which will take a large amount of damage from Brave Bird.