Well I would want to say Angelus or Darla because they're totally hot and y'know... really evil... but, let's face it - you couldn't trust either of those two to ever have your back. First chance they'd sell ya out or just abandon you to a mob scene in a stable out in nowhere...
So I'd probably choose someone like Spike. Less brains more brawns (no, I'm not calling him stupid 🙂) just a bit malleable. And loyal. Gotta like that in a partner... you could always trust Spike to rush in, guns (or fangs then) blazing to save your butt if you ever get in a jam!