I'd have to say Guilliman. Him and Dorn were the best alternatives for the Warmaster position. The reason i go for Guilliman is cause he was more coolheaded than Dorn while still being a great fighter and excellent tactician. The planets under his rule were utopias and he held the Imperium together after the Heresy. One other favorite is Magnus. I found his story kind of tragic.. how he was loyal to the Emperor until the "end", unwilling to turn against him even when the hounds of the Emperor unfairly came to burn his city for believing him a traitor while he was one his most loyal sons.