Dark Angel
My So Called Life
Dead Like Me
Freaks and Geeks
Pushing Daisies
Veronica Mars (Loved by many but underrated as a whole)
United States of Tara
Dollhouse
The Comeback
Tru Calling
Firefly
Ringer
Yes @ My So-Called Life and Freaks & Geeks. Although they are both praised, I feel like former is even more underrated, because the teenage show touching such serious topics like homelesness, violence and homophobia must have been totally groundbreaking.
Also, Six Feer Under, in my opinion. It was life-changing, but I feel like it's always being overshadowed by The Sopranos and The Wire