Hmm. That does raise the specter that some of these comparisons might not be too kindly, if we think them all out. Who wants to be Angron or Fulgrim, anyways?
Lion El'Jonson - Gav Thorpe
Fulgrim - Ian Watson
Perturabo -
Jaghatai Khan -
Leman Russ - William King
Rogal Dorn -
Konrad Kurze - Si Spurrier?
Sanguinius - James Swallow?
Ferrus Manus -
Angron -
Roboute Guilliman - Graham McNiell, the one who produces some great stuff and some, erm, drier sections of the Codex Astartes.
Mortarion -
Magnus the Red - Matt Farrer, indisputably.
Horus Lupercal - Dan Abnett, the charismatic Warmaster
Lorgar Aurelian - AD-B, the charismatic orator (well, AB-D could also be Sanguinius, the golden poster child hero of the fanbase)
Vulkan - Steve Parker
Corvus Corax -
Alpharius Omegon - John French and Rob Sanders. I will stand by this choice firmly. If you disagree you are plain wrong.
Then there's Pyroriffic, the Lost Primarch, of course, as our lovely S. P. Cawkwell.

... Sigmar - William King (again)
(I'm very, truly sorry that I couldn't resist, Ath)