YouTube video
YouTube video
YouTube video
YouTube video
This is a tough one...
I really want to say Frank, because hyperbole says he is more experienced, and IIRC he killed a man with a one-handed neck snap, but in that case he caught him off guard. In his straight up match with Urban, he seemed far more unrefined in his fighting technique than Mills. So I think I will say Mills here.
IDK. Like I said, in an earlier scene, Frank snapped a man's neck with one hand
Also, Frank can do this:
YouTube video
The one-handed neck snap can be seen here at 8:06, along with the CQB defeat of two other assassins.