This is a tough one... depends on if the chaos gods have a part to play.
If not obviously horus was the right choice....
Other wise I would have to say:
1) Magnus has the forward planning and learning ability to over come most foes
2) Russ hes the ultimately loyal general plus he would have the great crusade finished faster.
3) Sanguinus hes the best all rounded of all the primarchs and his brothers seem to respect him therefore would be a better combined leader.
Whilst Dorn and Guilliman are good choices I think the flaws in their personal character (anger and pride/snobbery respectively) would hamper their ability to lead effectively.