The biggest problem I have with putting Guilliman, Dornor the Lion in charge is that they aren't generally well liked by their brothers. The Warmaster is a position with lots of power in theory but it requires the consent of other individuals. Horus, who was both generally well liked and possibly the most charismatic of the Primarchs, had some problems getting some brothers on board and I can only imagine the struggle Guilliman, the Lion or (god forbid) Dorn would've had.
Specifically Horus is on good relations (relatively speaking) with the Primarchs most likely to be problematic. Mortarion, Angron and Night Haunter all count him as one of their few 'friends' and his relationship with Alpharius, Perturabo and Lorgar is a lot stronger than their relationships with others. I could see several of these Primarchs making a big issue out of Guilliman (Alpharius/Lorgar) or Dorn (Perturabo/Night Haunter) being appointed Warmaster and I don't think they have the ability to sooth those relations over (over even the interest in doing so).
From this perspective Sanguinious is obviously the second-best candidate (since Horus is out). His Legion's flaw could potentially become a big problem though. We saw them come very close to falling on Signus and I think if the gods had actually cared about that they would've be able to complete his fall.
Russ is right out, since he is generally not well liked or respected (Dorn, Guilliman and the Lion can at least claim that) and is far to headstrong to be able to manage relations with his brothers.
Another issue for Guilliman is that his loyalty is iffy. The fact of the matter is that Guilliman abandons the Imperium in its time of greatest need, that's not something I want to see in a potential number 2.
This leads me to think that an out-side-the-box candidate might be the best. Specifically I'll suggest Corax. Corax isn't necessarily well-liked or charismatic but he also isn't disliked and is fairly diplomatic. He doesn't have the raw charisma to win others to his cause (which is actually kind of a plus as it would slow a Heresy down) but he does have a good understanding of the psychology of his brothers and the patience to put it to good use. He and his Legion show no obvious flaw to be exploited by Chaos either.
"Look into my eyes, and see your death."
"Let them hate, so long as they fear."