Which champion would be most OP if granted stealth and why?
I personally think 
would be, considering his abilities individually can immediately put you at a huge disadvantage. Grant him mobi's + predator while stealthed and he'd cripple your entire team aoe out of no where.
not sure about how op it would be, but I think rammus would be fun
maybe Hecarim
or even sion
Or juggernauts I guess.
stealth? that's pretty vague.
+ stealth turrets.
Everyone's posting really fast champs that already have ways onto you if they're so much as on the same screen. Most of these wouldn't change much; they're already incredibly low counterplay as is. The guy that posted blitzcrank has got the right idea; cc skillshots when you don't know they're coming are incredibly dangerous, as is heavy burst that's normally gated by being able to space yourself from an immobile enemy.
Brand and annie would be my two big ones, but stuff like malz, cass, etc would be good too. Pretty much anyone with high damage, cc, and lowish mobility would be made broken by stealth. The whole idea being to remove as much counterplay as possible.
tl;dr: 
or 
Likely 
would be pretty high up there.
If you can't kite him to start, and he's able to get into a ranged carry's face immediately in combat/in a teamfight, he becomes way more problematic.
Alternatively versus anyone who doesn't have QSS 
enjoy going for a little ride
cuz if he did have it he could just come from behind the enemy team and dele-..ohh........wait
