Fiora's Blade Waltz
I really, really miss her old ultimate. Blade Waltz was one of the few league abilities (Alpha Strike among them) that realized the fantasy of a powerful sword technique being executed flawlessly. I really enjoyed it, and League's never been the same since without it.
Riot, I really hope you consider re-adding a similar ultimate. I really miss the feel of an ability like Blade Waltz. You could always make it a point blank melee-range skillshot that only activates it if hits, kind of like a super smash attack from smash bros.