I'd have to go with Nightshade. While he may be a bit slow, if you use him right, he can take out smaller enemies in one to two hits. The trick is: you have to get directly behind the enemy in his Stealth Mode, then attack said enemy with the swipe. It does over 300 damage normally. Plus, you can get lots of Gold this way.
Yeah, I discovered this last night. Nightshade all the way for me in any Nightshade vs Luminous discussion. Luminous is powerful, but pretty boring compared to Nightshade, imo.