Board Thread:Warhammer 40k General Discussion/@comment-112.207.234.222-20160507120912/@comment-2001:56A:7347:BB00:D597:A5C9:6D28:4E3B-20180815032543

It does state in the fluff that Horus and Lorgar both believe Angron might be the only one able to beat Sanguinious in single combat, pre enhancement Horus of course. Horus also believed that Sanguinious was superior to him it seems, believing he should have been made warmaster instead but that might just indicate Horus feeling of admiration for his brother and not ability indicating. Russ was seemingly designed to be the executioner, but in Betrayer Angron claims to have beaten him one vs one, though Russ doesn't appear to have been trying as hard as he would in a fight to the death.

I'd have to say that unless you let Magnus use full powers on his brothers, the other choice has to be Sanguinious, outside chance of Angron or Russ. The Lion and Kurze seem to go pretty much bout for bout over the course of the series, they have many fights, and though the Lion breaks his back before the trial Kurze wanted to be caught and put on trial to split the empire up with his testimony so who knows if that's a good indicator. The Khan would be a dark horse for me because he only ever has fought Mortarion in canon that I know of and it was a draw, which I guess makes Mortarion a dark horse too.