Crobat

Inner Focus
Prevents flinching.
Type Tier
Poison / Flying BL
Statistics
Min- Min Max Max+
HP
85
- 311 374 -
Atk
90
194 216 279 306
Def
80
176 196 259 284
SpA
70
158 176 239 262
SpD
80
176 196 259 284
Spe
130
266 296 359 394
Name Item Nature

Choice Bander

Choice Band Jolly
Moveset EVs
~ Aerial Ace
~ Hidden Power Ground / Hidden Power Fighting
~ Shadow Ball
~ Sludge Bomb
80 HP / 252 Atk / 176 Spe

Crobat is tied with Aerodactyl and Jolteon for Speed. Sludge Bomb has that nice 30% chance to Poison, so you can actually do something to common enemies like Tyranitar, even if you mispredict. Hidden Power Fighting doesn't allow max Speed, so you'll have to use Hidden Power Ground if you want 394.

Name Item Nature

Sleep Talker

Leftovers Jolly
Moveset EVs
~ Sleep Talk
~ Whirlwind
~
~
252 HP / 4 Def / 252 Spe

That's right, only 2 moves. The only way you will get this Crobat to work is by getting it put to Sleep, and also having 3 layers of Spikes. Sleep Talk will make Whirlwind go first, blowing away anything that's slower than Crobat—that's about anything. Not only can you do some massive Spikes damage, but you can also blow away things with Calm Mind, Curse, Swords Dance, and Substitute. Beware of waking up too early. Rest is theoretically an option here, in case they don't have a Sleeper, but then you have a 50% chance to pick Rest instead of Whirlwind.

Other Options

Its Special Attack is enough to hurt things like Swampert and Rhydon on the switch. Both are common switch-ins on Sludge Bomb if your opponent has no other resistance available. Quick Attack makes little sense since it has 130 base Speed, but it comes as a godsend when you are being swept by a Reversaler, as it can actually switch in thanks to that 4x resist.

EVs

Jolly. Max out Attack, and use at least 374 Speed on Crobat to outspeed Alakazam and Sceptile. The rest of your EVs should go into HP.

Max out Speed and HP for the other sets, putting the leftovers in Defense or Special Defense, it won't matter much.

Opinion

Crobat is best off with direct offense, in the form of the Choice Band set. It can finish things nicely with the amazing Speed, and thanks to the nice typing it gets 4x resistance to Fighting and Bug, among others. It just has a big problem with Steels, so when using it in standard, please use Magneton with it.

Counters

Pokémon that are resistant or immune to Sludge Bomb usually win against Crobat, including Swampert, Skarmory, Steelix, and Weezing. There is absolutely no chance that Crobat will every get past these, and others like Donphan and Forretress. Suicune's high Defense lets it take Sludge Bomb well, and while Milotic's Defense isn't as good, its Marvel Scale will increase it if it is poisoned. Aerodactyl outruns most Crobat, as they usually don't attempt maximum Speed, and it and Tyranitar (if Crobat doesn't have Hidden Power Fighting) smash it with Rock Slide.