I have two.
My first was Morgana. She was always a blast to play, skillshot-nuke-snare for clutch kills in fog of war on that recalling ashe, aoe-dot with a stacking mr debuff for pushing that you learn to predict how many ticks kill a ranged minion, dat shield for just walking away from ganking junglers and amazing shaq's-got-your-back moments for fleeing teammates, and the best teemo-finding ultimate in the books.
"stop! there's a teemo here."
*tons of damage follow
My second was mordekaiser. specifically mpen/tank morde. You just feel so good when you turn on ghost and just run into their team and start throwing out every ability you have just to see them scatter. or when you go lichbane, gunblade just for kicks and smash absolute face. and for everytime you stand at mid tower, they are long gone in the fog of war, and you hear,
"An enemy has been slain."