I have to second the call on Yuan-Ti. They are clever, vile and adaptable. Stylish as well. They also have lots of realated creatures, an excelent book about them and theyr allies with serpent kindoms and can be used in nearly all kinds of rolls: As "Demons" or their servants, dark sorcerers, psionic meanance, scheemers, disguised as human aristocracie, Lording over a state of slavery, old empire, lovecraftian horror creatures, mutated humen or even in hibernation in the high ice. And that's only the tip of the iceberg.
I think Undead are tricky. They gimp some important abilities and on the other side tend to be pushowers to a proper prepared or build party. They have some of the most player frustrating abilities and I've nearly allways experienced fights with them to be rather boring.