Some more light-hearted discussion of Primarchs is okay with me! Let's have a look at this...
1.Alpharius- Alpha Legion vs. Vulkan- Salamanders
2.Corax- Raven Guard vs. Lorgar- World Bearers
3.Horus- Lunar Wolves vs. Mortarion- Death Guard
4.Magnus the Red- Thousand Suns vs.Angron- World Eaters
5.Roboute Guilliman- Ultramarines vs. Ferrus Manus- Iron Hand
6.Lion El Jonson- Dark Angels vs. Fulgrim- Emperor’s Children
7.Sanguinius- Blood Angels vs. Luther- Fallen Dark Angels
8.Perturabo- Iron Warriors vs. Jaghatai Khan- White Scars
9.Leman Russ- Space Wolves vs.Night Hunter- Night Lords vs. Rogal Dorn-Imperial Fists
1/ Vulkan. Alpharius isn't the one-on-one type, in battle.
2/ Corax. He knew what he was doing in combat, while Lorgar's greatest feats are more to do with his charisma and oratory.
3/ Heresy period Horus wins effortlessly, of course, but I assume he's not the one we mean. Pre-Heresy Horus was certainly a brilliant fighter, but Mortarion has greater feats of strength and toughness ascribed to him. So I'll be controversial and bote for Mortarion.
4/ Angron, I think. He was an exemplar of hand-to-hand combat, while Magnus' talents ran more towards the mystical side of things, which won't help him as much in a fight.
5/ I'm thinking Ferrus Manus, as Guilliman was more of a thinker than a fighter.
6/ Jonson, of course. His skill in combat was near-unrivalled, while Fulgrim was not quite so focused.
7/ Sanguinius. Primarch defeats Space Marine.
8/ Tricky, but I think I'll go with the Khan. Just more ferocious than Perturabo, whose rather clinical approach won't serve him too well here.
9/ Russ, of course. Night Haunter is a nasty piece of work, but Russ was one of the greatest Primarchs in combat. Dorn makes a valiant effort, but he can't defeat Russ either, though he could give Night Haunter a challenging battle.