Lately I've just been buying champions that people say are good, or I think are powerful. But now I want a champion that is just plain fun to play. So yes, I would say having a ridiculous gimmick is a requirement (considering Hecarim for his hilarious tp homeguards). I want to be able to laugh out loud at a hilarious play I can make with my next champion, suggestions?
My picks for most fun champions are:
All of them are a complete blast.
It depends on your definition of fun. Some of the most fun I've had is playing with Jinx, but she's not exactly gimmicky.
To be honest? The stupidest thing I've seen in a long time is 11XX movement speed rammus with Q + boots of mobility homeguard + movement speed quints.
Sion's fun, but he's also a rage catalyst. Every game I play with him is much angrier than other games.
I also enjoy Ahri, though I wish she still had her damage amp.
Then there's always 
You can always buy 
or 
if you want to get reported alot 
Most likely one of the juking champions, Shaco, Wukong, Leblanc.
Urgot, his ult can do some crazy things.
You would be better off buying one that you have played and found fun if you are spending that much IP on them. Who knows if you find a champion someone else found fun, fun at all.
Blitzcrank is a blast. all you have to do is land 2 pulls in a match, and people will forever run from you from then on..its hilarious.
Fun... Yasuo.
Just spam E on the wave. WHEEEEE!
Any of the long range seigers.
Lux Ziggs Xerath
Sniping is very VERY satisfying
Zac in of himself is a gimmick enough.
