If you like live action comedy: Arrested Development, Modern Family and
Community are great shows to watch.
If you like live action serious drama\action: Breaking Bad, Game of Thrones (Most HBO shows), the Walking Dead, and Friday Night Lights.
If you like live action series that are less "serious" then those mentioned above: Supernatural, Castle, The Mentalist, Once Upon A Time, Stargate Universe, Entourage, Firefly and others.
If you like animes, you should ask someone else as I've seen none, but most seem to think Death Note is the shit.
Shows I havent seen, but people love: Fringe, Psych, Doctor Who and countless others, many whom are mentioned in this thread
And ofc South Park if you've never seen that, which I find unlikely.