I agree with a previous post... its hard to pick my fav... but my least liked were Reid, Mayweather, and Ho-She (as I like to call her)...
Reid was such a wuss compared to other Trek Security Officers/ Tactical Officers... Worf, Tuvok, Odo, Spock.. I mean those are big shoes to fill.. and Malcom didn't make the cut... I mean how many episodes had him having a break-down and saying how they were all going to die? I can think of like 5 off the top of my head... I think the only way they could have saved the character if the series had continued, was if it showed him slowly falling apart under the pressure and going insane...
Mayweather I didn't like because he was "THAT Character" of the "wide-eyed Ensign who is always optimistic and smiling" ... In TNG you had Wesley Crusher... in Voyager you had Harry Kim... I just hate these character on priciple (only one I didn't hate as much was Chekov from TOS, but he still got on my nerves)
Hoshi was annoying and useless... all the episodes that focused on her were just annoying... and she never really added anything to the crew... sure they could have done more with the character, but not too much more...