I just found out it's been a while since I've played Cassiopeia.
The best part is that there's one champion I have never played since the release of the mastery system, and that's Corki, so technically he's my last played champ.
Check it out here!
EDIT: I only now realised that last played champion and LAST played champion is exactly the same.
But "what's the champion you haven't played the longest sounds dumb"