A challenge! *cracks knuckles* Lemme see, now... Well, the Crimson Fox's boyfriend Renee CLAIMED to be 'the greatest thief in the world' back in JLI, although that could have just been hyperbole. The Italian version of Angle Man appears to be a pretty good thief - he's probably more of a mercenary, actually, but we've only ever seen him thieving, and he seems like more of a lover than a fighter. The Shadow Thief tends to live up to his name, and while the Gentleman Ghost isn't a thief per se, we do see him engaged in robberies more than any other crime. There are probably more, but those are all I can think of for now.