There's an interesting chapter in Scars where Fulgrim, Sang, the Khan, and Mortarion are sitting around after the parade on Ullanor.
Trying to pick one eventually comes down to a matter of personal preference, as the only definitive fight we've seen so far is Ferrus v a demon boosted Fulgrim, which skewed the result somewhat.
If I had to pick it'd have to come from primarchs that we've seen fight each other so far. Russ, the Khan and Fulgrim for me. Don't know how anybody could pick Horus, we've seen next to nothing of him in combat at all, never mind against another primarchs.
EDIT. Gonna have to add Curze. He's faced 4 of his brothers in combat, two of them at the same time, and walked away. He also nearly killed Dorn. He definitely needs to be considered a serious contender.