The Next Generation - Geordi LaForge. Cannot think of a more underdeveloped character in all of the series.
You are aware of the "character" of Travis Mayweather, aren't you?
I think when he wrote "all of the series", he didn't mean all of the Star Trek shows, but only all of TNG.
Okay, well, here's my list:
TOS - I really love all of the big seven, so I'll just say ... Pike. Seriously, thank god Jeffrey Hunter didn't want to do the show.
TNG - Wesley. Because when he started out he was plain annoying, and just when they had him actually become interesting they wrote him out of the show.
DS9 - Now, this situation is similar to the TOS one, I actually like them all, but ... well, Sisko got pretty annoying towards the last few seasons with the whole emissary thing, and Dukat ... I felt, the writers didn't really know what to do with Dukat, so they tried this, failed, tried that, failed again, went on trying. With Dukat, it really was the actor saving the character.
VOY - ... pretty much everyone but the Doctor.
ENT - Mayweather. Now, here's a token if you've ever seen one.
Producer A: "We'll need a black guy."
Producer B: "Okay, let's make the helmsman black."
Producer A: "Nice, but really, I have no idea what to do with the helmsman."
Producer B: "Me neither. Just have him on the bridge and in all the promo shoots of the crew."
NuTrek - That would be Sulu, and it'd be because of the actor. With every other casting in the movie, I can see why they were cast, most look enough like the original character, some at least got talent they showed in this movie. What made John Cho qualify as Sulu other than being of Asian heritage? He doesn't look much like Takei, he hasn't got a prominent deep voice like Takei, and he doesn't show much talent (to be fair, he really didn't get much chance, but still). Seriously, the guy who plays Sulu on "Phase II" is better qualified.