Most under rated, hmm.
Veronica Mars, cult following, maybe general people were put off because of the high school aspect but it was never remotely "kiddie" or teeny, it was just a awesome show, it should have been pushing ER style unmissable network ratings and not been cancelled for ages, and got its FBI seasons. While much appreciated, it should have been much much bigger.
Kings was almost completely ignored by everyone, Iain Mcshane, great cast, great writing, had god in it but wasn't "religious" in that way AT ALL.
Charlie Jade, fantastic series, brilliantly done, great to see a completely different location for a change, I never know anyone who has seen it.
Bored to Death, another show that seemed to have a cult following but was funny/quicky/brilliantly writen/epic cast that meant it should have been mainstream, loved and still be around.