Loki, for all the reasons you said. He’ll get you 100k Universal villain ABX, P-card, easy SL villain clears (if you’re not at 30/30). He’s also easily farmable.
2nd is probably War Machine. His IW uniform gives him a huge boost from trash tier to S tier. He can be used to clear reasonably high stages of WBU Proxima.
3rd is between BW and Satana. BW is trash without it, okay with it. But she’s nothing to write home about. Satana is in a similar situation, but she’s probably more useful for Universal Female ABX.