THE ORIGINAL SERIES: Sulu, just pretty dull and never gave a memorable performance. Hilarious that Takei thought he was a good enough character to base a whole series around.
TNG: Wesley really was pretty annoying in season one but got better. His mum was pretty boring with the only memorable Crusher episode being 'Remember Me' (but that's one more than some characters got, so she's not the worst. Oh and I know Eggs and others will disagree but LWAXANA TROI. Annoying is fuck and usually not in a funny way. That episode where she tries to teach Alexander to laugh...what the fuck was THAT?
DS9: Jadzia was kind of dull because Terry Farrell's kind of dull as an actress.
Voyager: Chakotay is the worst and you can't excuse Beltran's bad acting by saying the writers gave him shit, because everyone else got shit scripts too and they tried. Neelix, for example, was annoying and pointless but at least Ethan Philips was a good actor and gave some good performances. Kes was the most poorly conceived character probably in Trek history. Oh, wouldn't it be great to introduce a character who only lives for nine years? Umm, no, not really, that's a fucking stupid idea for a character. But she was hot when that evil guy was in her body. Harry Kim mostly sucked too.
Enterprise: Travis Mayweather was the least developed regular character since Sulu. Shran was a far beter character and he was only a guest star in less then ten episodes! Reed was kind of weedy and never got a proper centric episode that I recall. And as much as I like Scott Bakula, Archer was downright embarrassing at times in the first two seasons.