Utopia. It's a science fiction thriller that aired 2 seasons in the UK and is hands down the most amazing show I've evet watched. The cinematography. The acting. The visual design. The score. The story. All of it is phenomonal and if you have the means to track down this show, I highly encourage doing so.
It's not easy to pick just one... but some of my favorites are The Walking Dead, Game Of Thrones, X-Files, Twilight Zone, Lost, Star Trek: Deep Space 9, Xena: Warrior Princess, Buffy The Vampire Slayer and Once Upon A Time.