KOTOR 2 - Has quite a few pain points for me, usually when they make you swap characters, but by far the longest stretch that makes me reconsider replaying is the surface of Telos. It's just a bore until you reach Atris and feels like it takes forever.
I actually like Pergaus outside of the T3 part though, which I guess makes me a KOTOR 2 heretic. The atmosphere and mystery is just fantastically done.