All i can think of is Parrrley and Devastating Blow. What else can crit?

Blitz Rocket Punch apparently.

Not sure which else.

Siphoning strike can crit, but I think only the auto attack portion. If thats so, I assume same happens to power fist or garens Q.

Garen's Judgment.

spray n pray (twitch)

Anything that's based off an auto-attack can crit (ie: Takedown, Devastating Blow, Siphoning Strike, etc). However, I'm pretty sure when they do crit, it's only the auto-attack portion that gets multiplied.

Aside from those, I think it's just Garen's Judgement and Parrrley.

Poppy's DB can crit? I was under the impression that it couldn't.

DB cannot crit.

Devastating blow CAN crit.
Im sure.

Also, most of skills that use the basic attack can, but not everyone (warwick's skills cannot crit for example)