Incubus/succubus from Savage Species (same rules & abilities for both).
Want a powerful natural attack? You got it.
How about charm monster at will? Sure, why not.
Alternate form? Just throw that on there.
Perfect flight? Why stop now? Add it.
Mad stat bonuses? I thought you’d never ask.
Want a 1/10 chance of summoning a balor once per day? Have it! (BTW, the demon is not under any obligation to aid the creature that summoned it. But it’s still fun to try and convince them to).
Wanna play an incubus or succubus as a player? You can! It’s a total of 12 racial levels, and you get 6d8 hit dice, 8+int skills every other level, +9 natural armor, acid, fire, and cold resistance, electricity immunity, spell resistance, bonus feats, the ability to speak any language (tongues), and a few more things I didn’t bother to write down? Who would turn that down?