Of all-time probably "The Shield". Can't get much better than having Glenn Close, Forest Whitaker, Anthony Anderson, and Franka Potente as guest stars in a few of the seasons. You'll basically learn to never trust cops again though, still a riveting storyline.
24 (seen all the seasons, this season is ok so far, nothing will ever beat Season 2 though with the nuke in Los Angeles)
Damages (really awesome show adding William Hurt, Timothy Olyphant, and Marcia Gay Harden to the mix this season)
Prison Break (cancelled, but its time to wrap this thing up, Season 1 is the best season of any TV show I've ever seen though)
Heroes (although Season 2 sucked, Season 1 was great)
Lost (hasn't been a bad season yet, and Season 5 kicked off well)
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