I´ll admit I mostly know the Imperium and Chaos, but it has gotta be Sanguinius or Kurze.

Sanguinius took down Khorne´s strongest daemon. In Heresy novels, Leman Russ (who was able to wound Horus before he went over 9000 Chaos) says he think´s he could take anyone short of Angron, Jaghatai Khan (someone I feel like is always missed in these lists), and Sanguinius. Angron, who beats Russ in a 1 on 1, is either jealous or afraid of Sanguinius (Red Angel was mocking him compared to the Angel, ie Sanguinius) and their Legions were similar in tactics and big rivals. He also has something along the lines of the Black Rage/Red Thirst, but without any of the negative effects. Oh, he´s also telepathic and can fly.

Konrad is overlooked imo. In novels, his foresight basically allows him to see the fight happening 5-10 seconds ahead of real time. He´s psychopathic and brilliant. He´s also described as being quicker then any primarch and can warp into shadows (it´s somewhat unclear how this works, but he vanish into thin air and reappear elsewhere). He isn´t a warrior, he´s a killer. To the best of my knowledge, we never actually see anyone beat Konrad, he gives himself up in the Heresy novels.