My preferences are for Outsiders that have a definite "hook." By definition, they embody an ideal or a philosophy. The best ones are ones that embody one that is inherently interesting but also something you can wrap your head around. Most of the fiends do a good job at this: Demons/Tanar'ri (ravening destructive chaos), Devils/Baatezu (organized evil and temptation), and so on. They get into trouble when the philosophy is too opaque to make it readily accessible at the game table. Or, when they muddy it. For instance, I'd prefer Baatezu to be the tempting soul bargainer types, but succubi are Tanar'ri.