It's been a while since I've been so stumped of who to choose from a Nexus.

Duping Rintrah would be good but he's not as much in the frame.
Hyperion is, I think, objectively the better champion, and I do have a Cosmic AG (which I'd been saving for Serpent...) but Omega Sentinel seems to come up so often as a top choice for quests and content...
I do play Battlegrounds but not particularly seriously - generally just get to GC then stop.