It's a toss-up between Aether Wing
and Eternum
.
Aether Wing, first of all, is sexy and she's wearing a plugsuit (how does no one talk about this like they did Irelia before Sona took over, her butt is clearly there in all its glory, if not hidden a bit by the wings), but it also makes her E look incredible (and sounds loud as hell, seriously that's one loud auto-attack when her sword's all lit up) but Eternum Noc makes his ult WAY scarier ("OBLIVION." "PIERCE THE VEIL."), his taunts are awesome ("Beings of this plane are no threat."), and his model just looks incredible.
If I'm feeling down, I load up Eternum Nocturne, bring Clairvoyance to maximize the effectiveness of his ult and to interrupt recalls (he can live just fine without flash, his Q kites very well and E works well to break a pursuer if Q isn't enough to get away), and go to town. Nothing's better than interrupting a wounded enemy in a bush, them seeing the Eye of Death pop up above them, then that mechanical electric sound popping up, the screen turning red, and the thought, "Uh oh."
Also a really big fan of Winter Wonder
, Sorceress
, and Celestine
...though that last one might be trumped because she's never shown more purple skin. I mean, how has no one mentioned this.