The Simpsons have run for so long and varied so much in quality that they qualify as everything between overrated, underrated and legitimately praised/despised.

By the way, I consider South Park to be quite good, but the mixed reception makes it hard to say whether it's underrated or legitimately praised.

I also wouldn't know how to qualify my old beloved shows with Moore (Saint, Persuaders, Ivanhoe), which used to be popular but not much anymore, and are adorable but at times horrifyingly cheesy. Same with old classics as Danger Man (The Prisoner is special for its well-deserved cult status).

This thread seems almost the same as shows you feel are underrated. But I guess a show can be total crap, I like it, but don't care it's total crap.
I guess Family Guy would be in that category. It's stupid, but I love it anyway.
South Park I love and it's a good show
Copper...even though they decided to cancel the series it was a favorite of mine

Justified
Game of Thrones
Hannibal
Walking Dead
Hell on Wheels
Boardwalk Empire
Dexter - ran a little longer than it should have
Continuum & Doctor Who - Starved for sci-fi, these two shows scratch that itch.

It was cancelled but there was this show called "[url=http://en.wikipedia.org/wiki/Touch_(TV_series)]Touch[/url]". Lots of people felt good about the series. I think you'll find it interesting.

I want to mention Friends here too just because of <span class="bold">this</span>.

Edit: Also this.