TOS: None, They all worked for me.
TNG: Alexander and Wesley. Alex was too wimpy (I wanted him to be a rambunctious kid) and Wes was too... ugh.
DS9: Jadzia. She didn't feel like she had multiple lives.
VOY: Torres and Chakotay. Whenever Torres gets mad it's her Klingon sides fault and Chakotay didn't feel like an Ex-Maquis, but more like Janeway's sidekick.
ENT: I liked Archer in the beginning, who went down hill after that. Hoshi was mostly useless, Mayweather had no story (other than the clipper ship thing), T'Pol needed to figure out if she was Vulcan or not. Trip was just off and Reed was a bore. Phlox and Porthos were the only characters I was ok with.
TNG: Wesley - I hate child geniuses
I was going to say that but if Wesley was more like Doogie Howser than he would be fine.