Mine is Congressional, without question.
That course just seems to ... cheat me more than any others. Every other course has its ups and downs, good rounds and bad, but it "makes sense" more often than not.
Congressional though, just seems to rob you. Large breaks don't break. Slight breaks move the ball a foot or more (similar distance and ding). You make a couple really good long birdies and then you inexplicably miss short shots due to bogus break-physics and bogey them away.
It stands to reason I think that our least favorite course is the one we do the worst at, but even when I come away under par at Congressional, it feels more like I clawed some of the holes back after being repeatedly robbed.
So, how about everyone else? What is your least favorite course and why?