Voyager is the hardest one to answer this question with as, for me, about half the cast are underrated in my eyes.
1. I think Janeway is a fantastic captain [superbly acted by Mulgrew] who gets way, way too much hate. She is in the most difficult situation of all the captains and she is forced to improvise as she goes along. She manages to be strong, stern, tender, caring and rather amusing.
2. B'elanna : I think Torres is one of my favourite Trek characters and I wish more focus had been given to her. I love watching her struggles with her Klingon side and she is arguably the only Trek character who touches on serious mental health issues [she deals with depression, anxiety, insecurities...]. As gruff as she can be she is entirely likeable. I also found the romance with Tom to be Trek's best.
3. Seven. I am sick of reading people claiming she is just the 'borg-babe' because you obviously didn't watch Voyager if you claim that. Jeri Ryan was fantastic as Seven. Watching her explore [and regain] her humanity is fascinating and moving. Her relationshp with Janeway is wonderful and, generally, Ryan is stellar in just about every episode she is in. I find her acting wonderful.
4. Neelix is also massively underrated. I think he is arguably the bravest Trek character and I can't help but like him. The venom directed in his direction is...distasteful.
5. Kim also, while not spectacular, does exactly what he should. He is likeable and Wang plays the green, plucky young ensign well.