A new Warmaster would have to be Guillaumin, hands down, as the best overall strategic thinker and someone who lives the greater role the Primarchs and Astares should have than just fighting.
MEQinc, questioning Guillaumin's loyalty is somewhat spurrious given the circumstances... the Astronomican goes out, no one can get in touch with the Emperor, he knows at least two chapters (WB and WEs) are in open rebellion and actively trying to destroy him, and no other seeming loyalist chapter is doing anything more. Given the circumstances, he was doing exactly what his father would want him to do, carry on the Imperium in any way he could.
Regarding the charisma factor among the Primarchs, its obvious from all the HH books describing the pre-Heresy Primarchs relationships that Horus's charisma didn't slake the competitive nature of his brothers, regardless of their personal relationships with him. Alpharius/Omegon still chart their own course despite their history with Horus; Perturabo/Angron/Curze are all broken and accelerate toward their own dooms despite Horus becoming Warmaster; Sanguinius, Russ, Kahn, Corax, Manus, and Vulkan seem to be in full kicking-ass-taking-names mode before the Heresy; Fulgrim, Dorn, the Lion, and Perturabo are all otherwise primadonnas who think it should be them no matter who's selected; Magnus and Lorgar will both do their same thing - Magnus trying to stop the Warmaster being corrupted and Lorgar trying to corrupt him... they're both on their same headtrip for enlightenment.
I'm not a big Ultramarines fan, but Guillaumin is one of the few Primarchs who seems to embody the 'superhuman' mantle given to him without coming across as a whiney little pussy or a five-year-old having a tantrum at times. Papa Smurf is also smart enough strategically to actually see the patterns Lorgar was creating to undermine the Imperium if he was placed at the center of command. The Heresy would still have happened, but it would have been a 2/3 loyalist 1/3 heretics war.
The White Zone is for loading and unloading only. If you have to load, or if you have to unload, go to the White Zone. You'll love it... it's a way of life.
: Cigarettes in space?
: It's the final frontier, Nick.
: But wouldn't they blow up in an all oxygen environment?
: Probably. But it's an easy fix. One line of dialogue. 'Thank God we invented the... you know, whatever device.'