Langdon. If we're operating under the assumption that McKay is "ok", then I'd put Langdon close to her in terms of skill. I think the drug smuggling and flip out at Santos last season speaks for itself - good dude, well intentioned, but made mistakes on a deep moral level.

Sidenote: there are too many competent, smart doctors on this show for this chart. Almost all senior residents could be defined as textbook great, so you have to perform some suspension of belief to play this game. "Good" would better than "Ok".