The only two battles I remember having a really hard time with in Kotor2 was Atton's bar fight and those mercenary guys that you encounter not too long after you first get Bao. They were bunched together, I was a Kotor nub, and they wiped the floor with me the first time and I tried to just charge into them.
But on subsequent tries and game replays, they weren't hard at all. Everyone else, including bosses, were generally not very difficult.
Unless you count trying to use Remote to actually attack those monsters near the end game, just to see if you could do it.
So in terms of 'top foes' - for Kotor2 I'd have to say there weren't any that really stood out, at least combat wise. But story-wise, I like Sion.