Probably Jace. I should say though, that while he is certainly an intelligent individual and a skilled debater, I believe that his unusual worldview is the biggest part of what makes it difficult to debate him. Getting anywhere in such a debate requires a very critical look at ones own viewpoint, otherwise any problem with the minutiae will be focused on, even if it doesn't (or shouldn't) compromise the entire argument. This is not a bad thing, because those minutiae should be examined and frequently are not.

When preparing to debate most people, I would focus on the macro level, the large points, bringing the little details into it on an as-needed basis; Debating with Jace requires a focus on the micro level such that the best I've ever been able to accomplish is that we're operating from incompatible premises. He's made several of my positions look pretty bad- sometimes rightly so, and sometimes when they really aren't. But because of this, more than any other debater, he's caused me to re-examine my own positions. I'll admit that I rarely revise my position, but who does so frequently? It has been somewhat frustrating at times, but I'm a better debater for it, and Jace is certainly a valuable member of the community. On the whole, I like him, even if I wouldn't vote him to public office :P

GuitarGuy, Jace, and J-Roc

Jace is a difficult and thought provoking debate every time. I haven't had serious debates against many others.

I'd have to say thousandin1 .

JustIgnoreMe, Thousand1, Jace, and maybe a few others I can't remember. I don't think I have debated with JustIgnoreMe, but he always presents me with thought provoking questions. Helps me re-evaluate my views on certain topics. Thousand1 always leaves length worthy posts which also make me view certain topics differently. I've had a few good run throughs with Jace and he always gives good arguments.