nidalee's spear and her separate set of moves makes her fun.

i also find heimerdinger very fun because his skillshot grenade and his turrets.

olaf is hilariously fun. cuz when your low, you think "omg im going to die!" but then life steal and AS come together and you kill a few and run off.

Cassiopea is fun during the laning phase, akaki and ken are also loads of fun to play.

Akali, Kat, GP

Because killing stuff fun

Playing alistar duo bot with a competent blitz is funny as hell !

Xin is pretty funny too, basically he eats any squishy in a second, and his burst is on a really low cd.
Btw poppy xin duo lane is pretty hilarious.

Veigar also, the character itself and his ability to one shot any caster.