Bashir annoyed me to no end, even after he and O'Brien became friends! (Although their friendship was one of my favourite relationships on the series).
This will be unpopular, but the character I resented
the most was Ezri Dax. I felt they wasted too much time on a new character when they had really important plots to tie up by the end of the series. I won't say any more so as not to spoil Captain Kathryn!