Actually, Payback would be better than all of those. I'll take it piece-by-piece.
Faint Attack: Power of 60 (90 after STAB), Physical Dark type move, and never misses.
Dark Pulse: Power of 80 (120 after STAB), Special Dark type move, and accuracy of 100.
Payback: Power of 50 (100 after STAB, and most likely 150 power since Umbreon's Speed is really low), Physical Dark type move, and accuracy of 100.
Overview: Faint Attack never misses, but has a maximum power of 90. Dark Pulse can flinch the opponent, and almost never misses, and has a maximum power of 120, but is Special. Payback has a maximum power of 150, and you'll probably be getting that with it's low Speed, and almost never misses. I'd go with Payback.
I hope this helps!