Favourite have to be the word bearers, I love the idea that of all the reasons for treachery i.e. power, survival,why the fuck not (ANGRON) and the hatred of the Imperium, only the 17th chose Chaos because it is real, they are gods and so should be worshipped.
Also got a lot of love for the broken sons, Angron, Curze, Sanguinius and the like.
Absolutely hate the Wolves, a bunch of big headed, ignorant, hypocritical, self righteous, judgemental pups, the one legion who manages to eliminate 2 loyal legions both there own and the 15th (who probably would have been the most useful) in one fell swoop, even the 16th cant bost of an achievement on that scale (drop site massacre was a group effort). On top of that is the whole 'My sorcerers are not sorcerers', they are spirituals, they dont use the warp they use the land or some such shit.