Sheesh, that's a hard choice... well here are my favorites:

5) Jaghatai Khan
-Unites a bunch of disparate tribesmen to overthrow a freaking empire, pretty cool in my books.

4) Vulkan
-Hide in caves... or fight bad guys... hide in caves... or fight bad guys... eat hammer suckers! This guy grows up on one of the harshest worlds and hands the Dark Elves their behinds with a pink ribbon while doing it and all with smiths hammers, that's just plain cool.

3) Corax
-As others have said he totally owns a far more numerous and superior force with a bunch of half starved slave minors... twice!

2) Night Haunter
-Crazy, bat **** crazy, but cool as well. That's all I gotta say on the matter.

1) Leman Russ
-Ok so perhaps pretty cliche but still Russ was one of the few Primarchs to not just take over their homeworld and unite it, but establish a pretty well working government as well. Guilliman started with one and didn't have to really do anything, same with Vulkan, Jaghatai didn't really do governing, Corax didn't have time to do any really and so on and so forth. Russ started out growing up with Wolves and became undisputed King of Fenris... can't beat that baby.

However I wish there was more on Rogal Dorn and some of the other Primarchs, some of em don't really have much in the way of an origin story.