Regardless of who the last Primarch standing appears to be, the victor will always remove his helmet and reveal himself to have been Alpharius all along.
I have always wondered what the fulgerite did to Vulkan. Did it heal him but take away the eternal life thing. Otherwise Vulkan is dead. Or did the fulgerite just bring him back from madness etc. Personally I think it healed him and he was no longer a perpetual afterwards. So this is a list of alive or dead that I know of in another post I will do a list of who I think will win both in 30k world then 40k Alive or dead. Chaos Horus- dead Konrad- dead Pertorabo -alive Fulgrim- alive Mortarian- alive but smells like a corpse Angron- alive Magnus -alive Lorger - alive Loyalist Gulliman - alive and kicking Russ- suspected alive Lion- alive in the rock Dorn-alive suspected missing arm Vulcan- alive till 33k suspected still alive as perpetual Kharn- suspected alive Corax- suspected alive Sanguinius- dead Ferrus- dead Unknown Alpharius- suspected dead omega not lives
After discussing this at length the other day, decided to do a test with the Forge World Horus Heresy books, this was prior to the release of Inferno and therefore didn't include Russ, Magnus, Sanguinius or The Kharn. We decided that each match of our tournament would consist of a single round of shooting each, just for the hell of it, followed by the initial charge coming from the one that claimed first turn (Initiative + D6). End result came down to Horus and Lorgar (Transfigured), with Horus claiming a very close victory due to the Talon whittling down stat lines. Lorgar was a surprise to get as far as he did, but divination can give you one hell of an advantage one on one, and while Transfigured you're able to pick your powers, and harness on a 3+. Lore wise, depends on when the confrontation takes place, if it was before or after the return to Davin, and the imbuing of Horus, as nobody really stands a chance after that. But in the series, if I remember correctly, the Primarchs spoke mainly of Sanguinius, Russ and Horus being the best Combatants, can't remember if Angron or the Lion were on the list, or if they were excluded due to their...... Flaws. Magnus was referred to as the most powerful and strongest by many of his peers, but isn't much of a duelist, so it also depends if you're insisting fair fight.
Horus. Ironically there's a non-Primarch who wipes the floor with all the other Primarchs. Iron Father Smashfucker is king.