Akali's, Yasuo's, Zed's, and Katarina's passives can become...Frustrating.
But speaking for non-passives, I mean, there's a lot to pick. But the most useful non-ult?
's E earns that spot. Hands-down, no contest. Give her a Nashor's and a Runaan's and she'll be tearing through the whole damned rift with that E.
's W can ruin someone's day pretty easily, thought it necessarily has to be combined with one or two of any of her other skills.
's E can make you crazy.
's Q is one of the most annoying harass moves in the game, if only because it's an auto-hit with a lot of damage and a short cooldown.
's W wins teamfights. It goes especially nicely if she's used it on an ally champion during a pursuit.
's Q and W are both often underestimated. The Q has incredible range, excellent AoE, goes through literally everything (you can hit people from the jungle at their own turret with it), and has the beautiful effect of making whomever it touches Nocturne's new target, increasing his damage and movement speed, along with ignoring unit collision for a pursuit. You know, in case it didn't already kill them if you go AD Noc (which you totally should, it's a way more effective build than AS Noc). His W can block any spell, and that includes, uh, everything. Is an Ashe/Jinx about to launch their ult into your team? Get in the way and pop it. Nullified completely. If it can be blocked, W can not only nullify it, but double its passive attack speed bonus, which is 20% at level one. Not too bad.
Additionally, Nocturne's W makes 1v1ing the dragon way easy, since that fireball now counts as a spell. (Yay!)
's Q is not only staggeringly powerful, she can move and cast it at the same time, meaning anyone chasing her is going to continue to get nailed with it. Ganking anyone who relies on skillshots is a breeze, as you literally run circles around them while they try to hit you. If they can even get a hit in, because Eve sure does a lot of burst damage if you let her get near you without a way out.
's E. It's the primary reason, I think, why people hate going against her so much. It makes it almost impossible to chase her, and all the while her W is about to come off its cooldown so she can kill you and get all her health back.