Richard Dawkins, Sam Harris, Christopher Hitchens, or Dan Dennet? Go!
Richard Dawkins: Big fan of his science, not of his rallying cries for mockery.
Sam Harris: Super smart guy, a bit weird, great debater.
Christopher Hitchens: Remarkable mind, best of the very best at debates. He will be missed.
Dan Dennet: Great at making complicated things understandable. Not very good at being lead or solo debater, but an excellent resource as a backup debater in group debates
For me it's Dawkins, Hitchens, Harris, and Dennet.
Dawkins first because I also studied Evolutionary Biology and I was just naturally drawn to his works on evolution and atheism.
Hitchens is a very close second just because of how eloquent he was and the way he could masterfully match anyone on the debate stage.
Harris third because I always enjoy reading his philosophical perspective.
Dennet fourth because (as ZenBear said) he was just never that active or involved. At least, he didn't seem like it to me.