The best I've had is Forge - Kc/Forge/Fortress/Fortress niche, but one of those "wait you can do that?" moments for the opponent.
I'd make a nomination for Rogue. First, it allows you to utterly hose the other guy's deck - trashing gold without letting them get silver or 4 coin kingdom cards. Second it lets you swipe key cards (like say their Rogue's) so not only are you hurting them, but you are building up your own deck. Lastly, it doesn't allow the other guy to pile deplete like when you chain Sab a province.