TOS: McCoy. He acted racist (or speciest?) towards Spock, I hated that. I get that he and Spock were supposed to be the little logical and emotional voices on Kirk's shoulder, but McCoy attacked Spock on a personal level, he always had an "F*** you and your logic, you green-blooded heartless alien!" attitude. It wasn't an unrealistic portrayal, but it made me uncomfortable.
TNG: Riker. What a self-entitled, pompous ass! He turned down command after command because he wanted the Enterprise, he seemed to be waiting for Picard to croak and assumed that Starfleet would just hand him the keys to the flagship because ... uhm ... because the captain's chair was already used to his ass? Seriously, what was his reason for thinking he would even be considered for the job?
DS9: Jadzia. She was quite boring, she never bothered me, but I didn't care for her either.
Voy: Neelix, Chakotay, Kim, Tuvok. Hated all of them and wish they had dumped these four instead of Kes. The cast was too big anyway, reducing them to Janeway, Paris, Torres, Doctor, Kes and Seven would have been better and would have allowed to add another interesting male character.
Ent: Archer. He was incompetent, I drove me crazy when he wouldn't listen to T'Pol (or common sense!) and do something stupid episode after episode.
I haven't seen Enterprise in years, but that's how I remember season 1:
Archer: What do we know about the Blorp?
T'Pol: They hate outsiders and almost killed the last vulcan crew that tried to make contact, they have no ressources or knowledge that are of any interest to us right now, we are also supposed to arrive at the colony thetabeta VI to deliver food for starving orphans in 6 hours.
Archer: You know, when I was little my father told me stories ... blablabla ... dreams ... humans can do anything ... blablabla. I say we land right in front of the king's palace and say Hi, they probably just hate stupid vulcans, humans are fun!
*6 hours and 3 dead crewmen later Enterprise arrives at thetabeta VI with plasma leaking out of its port nacelle*
Archer: Well, that was close, but we made it, no orphan will starve today! And we learned an important lesson, the Blorp don't like us very much.
T'Pol: I tried to warn you ...
Archer: Next time, warn me harder! It's all your fault because I thought you were just trying to stop humanity from reaching its full potential. *pouts*