Of the list presented, Neelix, absolutely.
The man had a tragic backstory, scrounging and having to pull some dirty tricks in his past most probably just to survive, battling with his inner demons, having seen horrible things such as the Metreon Cascade.
When he finally has a lucky break, and finds this company of kind people living in a community based on mutual trust (on Voyager) he desperately tries to ingratiate himself and make himself useful, but with limited success. There is a significant character evolution arc from The Caretaker where he's just this cynical 'what can I get out of these people, I'll just as easily double cross them when it fits me' guy, through Fair Trade, where he is halfway (seriously struggling with his conscience but still not quite able to believe these people won't let him fall no matter what) to the dependable person he'll ultimately become near the end of Voyager, where even Tuvok honours him when he leaves.
It's just a pity they mostly used him for (bad) comedic relief. But even that fits in his psychological profile. I would've loved to see more episodes of the calibre of Jetrel, Fair Trade, Mortal Coil, exploring deeper dimensions of Neelix, though.
I also love Nog and Garak, but I don't think they're underrated.
