I have to say that all VOY characters, for the most part, had lots of potential (except for Neelix, who was a bad concept from the start, and Kim, who was an uninteresting concept from the start) but were hurt by the inconsistent writing and the writers apparently just not caring enough to put serious character development above 'whatever we need to make them do in this week's plot'.
All in all, the characters I liked the most were Lon Suder and Lindsay Ballard, but out of the main characters, from the best to the worst, I'd rank them something like this:
1st place:
EMH - Trek writers always found it the easiest to write for non-human, outsider characters, so it's not a surprise that the best developed character on the show was the sentient hologram; and in addition, Robert Picardo was great in the role. The only thing I dislike about him is that, in the last couple of seasons, he started to be overplayed as a comic character. Before that, I would have perhaps put him in my top 10 Trek characters (which I am afraid to say probably doesn't include any VOY characters).
2nd place shared:
Tuvok - Tim Russ was excellent and I always enjoyed the stories that focused on Tuvok, which included some of my favorite episodes of the show (Meld). The problem was that he was underused because the writers just couldn't think of much for him to do outside of those 'a Vulcan loses it' episodes.
Seven - Again, the Trek writers find it the easiest to write for the 'outsider learning to be human'. Despite the silliness of the catsuits, she turned out to be a very interesting character who brought the much needed conflict and different perspective in seasons 4 and 5. On the strength of those seasons (and episodes like "Prey") I might have given her shared 1st place and considered her one of Trek's top 15 characters, but as the show went on, I found that she was not just overused but her characterization became a bit too lazy - they were overdoing the Borg catchphrases and all, which became annoying especially since Seven would be "learning how to be human" in one episode and then acting ultra-Borg in the next. It didn't help that the portrayal of her attempts at individuality and romance was so awfully generic and boring in "Unimatrix Zero" and the end of season 7 with the unconvincing romance with Chakotay. It made you wish she had stayed Borg, if becoming "human" meant becoming duller and actually losing any personality she had had. Something went wrong there with the writing. If you're going to portray human individuality, maybe it's not the best idea to portray your character's dream place as the most generic, dull landscape possible where, ironically, people who want to be 'individuals' walk around all sharing the same dream. Or have your character's "romance" in the shape of dull, bland Hollywood 'romantic' scenes (complete with corny lines and the generic 'romantic' music) with a character she shared no chemistry with.
Janeway - Mulgrew gave her a lot of presence, charm and authority, but the writing for the character was too inconsistent, and I never felt that it gave her the depth that a main character should have. I never felt I really knew her, her background, her soul, her motivations, like I knew Picard or Sisko. It's as if there is a great character lurking there somewhere, who could have emerged only if someone cared to actually envision a character arc for her and follow through with the serious and consistent exploration of her motives and decisions throughout the show...
Tom Paris - I thought I was going to find him annoying, but he turned out to be one of the few characters with some actual development on the show, his relationship with Torres was well done, and I often found him likable simply by virtue of being the one character who acted like a real human being would, rather than spouting some dubious ethics or other. (A good example of that is "Nothing Human", one of my least favorite episode, where he is pretty much the only character whose behavior makes sense and that you don't want to slap for being an idiot, since he has the simple human reaction "I just want B'Elanna to live".) OTOH, the funny one-liners, car obsession etc. were a bit too much at times. He became one of the victims of the unfortunate turn in the last couple of seasons, when the show seemed to become more similar to a sitcom than to a drama.
B'Elanna Torres - In the first few seasons, I would have put her far below Paris and Janeway. I never cared much for the concept of the character. I tend to like many angry, feisty female characters, but it's a big difference if someone has a good reason to be angry and traumatized (like Kira) rather than just hating their origin. I disliked the way they made her constantly pissed off and explained it by her being half-Klingon (and it was especially grating and silly in "Faces", with its implication that B'Elanna's human side would be a complete wuss, and only her Klingon side was tough and angry). The reason I now rank her character higher is that she was another character who actually grew and developed, and that she was eventually portrayed as someone who was traumatized by her family circumstances (her father leaving) rather than just an example that mixed race children have to have an identity crisis and self-loathing, or that biology determines one's character, the things that I always found problematic about the character. In that way, I should perhaps rank her higher than Janeway, since she actually had her flaws portrayed and dealt with and had an actual character arc.
Kes - I'm not a big fan of super-sweet cutesy characters, but she had potential, what with the telepathic abilities and all, and while her relationship with Neelix was awful and unconvincing, and the Paris thing was just meh, her friendships with EMH was one of the best relationships on the show. If they had kept her, she could have made a great trio with EMH and Seven, maybe mentoring both of them.
Now it gets a lot more negative than positive:
Neelix - in the first few seasons, he was my least favorite character on the show and perhaps in Trek in general. Annoying, stupid, ineffectual as the comic relief, silly rather than funny, and the only thing less believable than his 'romance' with Kes was his position on the ship, and unfortunately, unlike the former, the latter never went away... The saving grace, however, were his centric episodes, which were, amazingly, all great and gave him a lot more depth, focusing on his background and the traumas and tragedy he masked with his outward cheerfulness. He was a great character whenever an episode focused on him, and completely unbearable in every other episode.
Kim - there isn't much to say about him, he just never went anywhere, never had any kind of development, remained the same as he was at the start. He might have been somewhat likable, but too dull and rather useless. The only reason I might not rank him the lowest is that his character never had any particular potential to begin with, unlike...
Chakotay - looking at the character description, he could have been such a great character, but he turned out to be such a disappointment. They botched everything about him - he wasn't convincing as a Maquis leader, he was dull and pointless as an XO, his spirituality came off as superfluous and a lot of nonsense, he got completely pacified and pushed into the background too soon, he was rarely allowed to voice any sort of disagreement with Janeway and was made to fold quickly even when he did, e barely had anything to do in later seasons except to sit on the bridge with a smug grin and smile at Janeway's quips, Beltran obviously lost any interest and his acting was wooden... He could have been this stoic, strong, spiritual outlaw leader with his own ethics code who presented a different perspective to Janeway's Starfleet principles, but it was all botched on the show and he became nothing more than a tall, good-looking block of wood.