Just off the top of my head, I'd say Sideous. I think Darth Plagueis is possibly the strongest Sith that's ever lived (excluding members of the Sith species), and after training under a guy like that, Sideous is the most serious contender for the #2 spot that I can think of. I haven't played KOTOR 2 in years though, so I don't quite remember everything about Nihilus. I actually just beat the first one last week, and I've been planning to play the second one again sometime very soon, so perhaps after I'm done with that, I'll be better prepared to make an informed decision.