In my opinion the top three underrated shows are:
Arrested Development: This show was absolutely brilliant! Great humour, unique premise, amazing cast - just brilliant.
Veronica Mars: Definitely a "Better than it sounds" (
link)show - the writing on the show is perfection.
Futurama: This show is HILARIOUS!!! All the characters are money on this show.
Honourable Mentions: Buffy the Vampire Slayer, Scrubs