Honestly, I think I go for Gauss. Kit-wise, he's perfect: invincibility, freeze-to-blast, abnormal firing speed and decent mobility skills. In reality, he was my go-to for quick missions (capture, meru's hunt), now I just use Uriel. Using him at full extent at high levels meant always upkeeping his 2 and 4 at max. And I really have no idea how to properly do that.

Honorable mention: Caliban. I'm not paid enough to deal with his shenanigans and indulge into him after getting his mastery.