There's a few different issues being combined here.
How well thought through was the character at the creation process?
How well written was the character?
How well did the writers follow through on the original plans?
How well was the character performed?
Star Trek has had several characters who seemed like good ideas at the beginning, and got at least some good writing early on, and whose actors gave good performances in the role, only for the writers to just give up on them.
Ro Laren is a prime example. "Ensign Ro" and "Pre-Emptive Strike" are two of my favourite TNG episodes. In the episodes between those two she didn't get much more to do than Ensign Gates, who was in 48 episodes of TNG. But Michelle Forbes was such a good actor that they wanted her for DS9. She wasn't the problem, the character wasn't the problem, the writers were.
Travis Mayweather had some interesting backstory that was followed up on... twice, maybe? I'm not going to get into whether Anthony Montgomery was a good actor because he didn't really have much of a chance to show whether he could act. But he was on Enterprise. Voyager and Enterprise were the "we came up with so much cool backstory for the characters and we're going to ignore it all!" shows. The producers or the writers decided to make Enterprise more TOS-like by making everything about Archer, Trip, and T'Pol, and Mayweather, Reed, and Sato suffered as a result.
So there were a lot of wasted opportunities, including Saavik, Kes, and many others. That doesn't necessarily make them the worst characters. To be one of the worst characters, I think you need at least three elements: a badly conceived character, badly written, and badly acted. Ladies and gentlemen and others, I give you Jonathan Archer.