I won't mention humans. Of course, humans can do all the things we imagine, cause it's us humans who imagine those things, and we also imagine all the other races. Neither will I say Demons and Devils in general - sure, tehy are powerful and will torture you for centuries, but I like another breed of evil.
Drow are good - especially Vhaeraunans, who live on the surface and have no big problems with daylight. It's just great to have the players venture into the woods in broad daylight, seeking the evil dark elves with lots of daylight spells for when they find the cave entrances to their underdark lairs, with spells against spiders, and with the plan to "take out the females first, for they're the spellcasters, healers and commanders" - only to have them attacked in the forest, in broad daylight, with male commanders and spell-slingers, but without a single spider. There's nothing more rewarding than the look on the players' faces when they realize that they are caugt with thair pants down, and with many precious spell slots wasted
Illithids - "Thinking is free" my mauve a**. Add the mind-flayers' brain-eating habits, and you know what will make heroes pale with as little as mentioning. (Be sure to check out the Mindwitness - Half-Illithid Beholder in the latest "In the Works" column on the wizards web site)
Dopplegangers - For you can't trust noone any more as soon as they hit the scene.
Succubi - Especially if the players still have that "beauty equals goodness" error in their thinking. I remember when someone on these boards told a story he pulled on his party: They defeated a Succubus, but she got away. I think they roamed the lands a little longer, but eventually they returned to one of the characters' home, where his father lived. They discovered that he had a new wife - and to their horror, they found out it was the very succubus they crossed. At a revel she danced with her new son-in-law, told him that his father loved her, and that they should go on and expose her. This would result in the father being exposed, too - as someone who traffics with demons. Then she told him that she bore his unborn brother/sister and danced away...
One of my favourites are the fey'ri - Succubi-bred gold elf tieflings out of the Forgotten Realms. They're both great villains and a great PC race for evil characters. Their abilities (all weaker versions from succubus abilities) make them perfect spies. Put in some sorcerer and rogue levels, and you're set up. They can be found in Monsters of Faerûn (as monsters) and Races of Faerûn (as PC race). They have a comparably low CR, so you can use them even at low levels, or add the classes you like with many levels.
