Wire. Hands down. Or. Battlestar Galactica
Double Like the above.
If you dig science fiction,
Battlestar is the way to go. Miniseries Pilot + 4 seasons. And series is complete. Also 2 extra movies concerning same characters if you are interested.
If you are into reality/police drama (with humor),
The Wire (though dated now, amazingly enough) has some of the best acting you'll find anywhere. Might have to give it a few episodes to get into the rhythm.
If you are into Politics:
House of Cards is awesome (though only 2 seasons in right now).
If you are into historical European stuff:
Vikings is really good. Also only 2 seasons in.
If you want British comedy/drama that most people haven't seen, but is actually pretty decent:
Sherlock. Only 3 episodes per season, but 1.5 hours per. Three seasons total.
If you like tech nerds,
Silicon Valley is absolutely hilarious. Only a handful of episodes into the first season though.
Some people swear by
The Walking Dead. I've watched every episode, and think it's quality is not so hot right now. Though the last half of the latest season was fairly good.
Also hear good things about
Orange Is the New Black. I just started so I don't have any impression myself.