I think Buchanon is way off in his hypotheticals. German would not have crushed Russia and Britain would never have sued for peace, regardless of whether Churchill was there or not. Britain was not going to fold once they were attacked - no way.
Plus, these hypotheticals also make Hitler suddenly reasonable. He was a maniac and was going to spread his forces too thin no matter what. Even if somehow he established victory over Britain and Germany (which I can't see anyway), he was not going to be satisfied. He was going to keep pushing. Plus, no way he could hold those nations for long. Even his hold on France was on the verge of crumbling from within. Germany was destined to fail eventually. It didn't hinge as perilously close as we like to portray.
All that said - I guess Churchill is a good choice anyway.