TOS: Yeoman Rand. Grace Lee Whitney had presence & was gorgeous, and should have been given real things to do. But as a character, she was there just to serve Kirk, and it seemed a little demeaning and useless.
Chapel seems to be a popular pick-- The actress was there for obvious reasons. But her minor, unrequited love schtick was a cute side thing that served its purpose. Chapel never meant to be much. Majel Barrett looked much better in "The Cage"- the blonde 'do didn't do much for her.
TNG: Geordi LaForge. Waste of a good actor, and to lose those expressive eyes behind the Visor was a loss. Geordi was at turns bland, moody-- his initial behavior toward Scotty in " Relics" was a turn off. His inability to interact with women was bizarre- that's was awkward teen behavior,from a grown man. LeVar Burton had so much more to give than the writers gave him.
DS9- Jadzia Dax. The actress & writers got a handle on the character a little too late, then she left. Just didn't buy her character as an "Old Man"- just because you say in the script that she's a wisened Trill with all these lifetimes of experiences, doesn't mean it works onscreen.