There's certain characters that just bug me. I find TNG episodes that centre on Mrs Troi out-and-out unwatchable. She's a lot more tolerable on DS9 -- the lack of any "Deanna needs to get laid" storyline really helps, and the little 'romance' with Odo is awfully cute.
I find the DS9 episodes that centre on Quark and the Ferengi pretty difficult to get through as well. Armin Shimerman's great, so too Max Grodenchik, but I like them as supporting players far better than starring cast. I can't really pin down why. I generally like Wallace Shawn ("inconceivable!") but I just really can't get into the Nagus episodes.
And with Voyager it's Kes. Uggghhhhhh Kes.