Huh... depends on the context, because:
In terms of gameplay, my favourite are the Dark Angels, or at least they were awesome in 4th Ed. Bikes with a 48" first turn move and Terminators deep striking in turn one without scatter around the bikes. Pure pwnage.
In terms of aesthetic, nothing beats the Iron Hands and the Morlock Terminators. I can't for the life of me find that Golden Daemon entry for 40K squad that was five Morlock Assault Terminators, stunning piece.
And in terms of fluff, they're mostly as mad as each other, but I like the Legion of the Damned.
Least favourite would be the Smurfs, but since they're public enemy number one for most Marine players, the Grey Knights (especially now... Mat Ward, you damned butcher...)
Not that any of this matters, because I'm an Eldar to the end. Bring back the 'Quins...