Sometimes you just have to troll when you're playing with some exceptionally gifted ragers in LoL. So hilarious. I don't usually intend to killsteal unless i'm playing the AD Carry in which case they can all fuck right off but when i'm using Amumu's awesome AOE teamfight potential to damage every single one of their team at once (you can just cry on them, it's so fun) some people just fucking rage for no reason... Sorry I just won you the game asshole. A good 'Mumu does that y'know?
I've also started to spam my laugh (bind it to your keyboard, it's amazing) whenever I have a chance now, especially when it's a super cool/noticeable one like say Rengar's or Lux's or Lulu's. It's just funny. I once stopped running from 3 guys during a chase, stood next to a wall, waited them to come close, laughed for a good two seconds then Arcane Shifted over it for teh lulz. Fuckin' Ezreal...
Kha'Zix is the best though, I would say if all his stuff is off CD and he's level 16 he will not die to a 5 man gank. His Leap takes him way too far, he has three speed boosts that turn him invisible and has a huge AOE slow he can fire behind him to stop their chase. Plus Flash natch. I was even slowing myself down by spamming the laugh he's just that good. Underpowered/useless as hell compared to other junglers/tops since his main damage mechanic is unreliable and he is squishy as hell but damn he is so fun.
Listen to the sound effect on his Leap... He pretty much is laughing at people.