Yeah, Uriel is the contender. I'd personally recommend Uriel over Svarog, though.
Svarog deals amazing damage per turn, and buffs the team with 40% Assault. Later, she gets replaced by Haruhi for perfected groups.
Uriel deals 2m burst damage when you need it. Combined with Full Burst, that's 3m damage from a single Unit, even off-element, even for players who don't have their own Belial. That's no fucking joke. A lot of content in the game is entirely trivialized by something like this - that's something Svarog simply doesn't do. Uriel also doesn't get replaced by anyone later on - however, it should be noted that for omega space whales, they don't really need Uriel either, since they reach 2m bursts with all Hime anyway. For everyone else, Uriel is simply a superb unit.