With a thread title like this, it really should be in General Trek Discussion. If you wanted a thread saying "Neelix is the Worst Main Cast Character in All Star Trek", you should have simply done that.
My vote for worst main character is Chekov. It's nothing against Walter. But the character was created to appeal to youth at that time. I was a youth at that time (16) and considered Sulu the already existing youth character. My favorite character for that show though, was Kirk. I resented Chekov simply because he was an obvious attempt to pander to the youth market.
For Voyager though, I'd have to say the worst character was Kes. Again, it's nothing against Jennifer. But they created a character with a variety of unspecified mystical powers who kept getting more mystical powers, and they still couldn't figure out how to write her successfully even after 3 years. She was always the least interesting character for me.