As popular as it was, I still think Cheers is the most underrated tv show of all time.
It was on the air for like 10+ years, won tons of awards, was loved by many, and is generally considered to be one of the greatest shows (definitely sitcoms) of all time.
And you think it's underrated?
I'll go with the obvious answers of things like Freaks and Geeks, Arrested Development, etc. But at least those are generally loved these days.
I'll go with SportsNight. Just a great show with awesome, fast-paced dialogue.
Also, if we're talking about underrated as pretty much "ignore by the masses of America," then I'll also submit Community. Sure, there are some misses here and there, but all in all it's pretty solid, and its hits are fricking grand slam homeruns.