I think Torres is the most underrated, a really strong character who made a lot of episodes good or better than they otherwise would have been and had only a few misses. Close is Chakotay (not a great character but a good one) and then Seven of Nine (had a lot of interesting, enjoyable growth although not as much as she should have for how much focus she received).
I have to say, Kes was really underdeveloped/neglected and Kim sure had a lot of bad episodes.
I have to say, Kes was really underdeveloped/neglected and Kim sure had a lot of bad episodes.