Inferno is my favorite faction. All the demons (not hellish beasts, but actual demons) are buff and hot!

My opinion on the units:
Imps/devils/little devils - cute, but not very impressive.
Horned demons/fire demons/jumping demons - buff, with dry muscles and a hint of rage. Awesome.
Hellhounds/cerberuses/hounds - I don't like them.
Succubi/demonesses/seductresses - horned girls)
Hell horses/nightmares/horrors - I don't like them.
Cave demons/lords/spawn - my favorite creatures in the game. Buff, fiery, and hellishly hot guys. The best in the game.
Devils/archdevils/archdemons - pretty cool guys with staffs.