My favorite role is assassin I love diving in and killing people in an instant. I like playing mages also and sometimes ADC. The champs I have in the 4800 ip group our Orianna,MF,Lee Sin,Wukong,Vladmir,Talon,Nocturn,Malz,Kennen,Fizz and Ezreal. I don't mind jungling either.
I'm looking for a champ that is fun to play not op. I am thinking of buying Brand,Cassiopeia or caitlyn (maybe vayne). I don't know who to buy.
From what you called out, I'd say look at:
(lots of combos, capable of diving and killing)
(super flexible between damage and tank, and I think he's really fun)
(mage/assassin, pretty fun and flashy to play)
(kind of an ADC/assassin hybrid - pretty fun if you like mechanically intense champs)
Most fun*
Riven hands down. I'll admit she isn't fun to play against, but you'd be hard pressed to find a champ funner to play than Riven, at least in my opinion as a fighting game player.
BRAND!
Really, he's very fun.
My vote is for 
think hes 4800 ip and hes fun
Seriously. He's so easy to play since the answer to every solution is "more ghouls."
You'll be like Christopher Walken in that SNL Sketch.
"GUESS WHAT! I got a fe-vah, and the only prescription... is more ghouls!"
brand is crazy fun in urf but is kind of mixed bag normally he is fun, however in midlane he is outclassed by a few champs and as support if your jungler and adc are not competant you will be screwing them over sometimes.
Dammit xD I bought cassiopeia since she looked like a lane bully but now really want riven haha. I'm gonna buy her next thanks guys :P
try rumble out maybe??
