My favourites? Not too hard to decide, unless I'm capped at 5 or something.
Definitively the top one would be «24». Real time, Jack Bauer ass-kicking, incredibly good side characters, and all those adventures.
I also like NCIS (as someone above said, "incredibly epic", as well as dramatically hilarious), CSI: New York (the best CSI without the excess of social commentary and Caruso's one-liners), Dark Angel (well-themed fanservice, sadly cancelled), John Doe (also cancelled), Medical Investigation (ALSO!!!!), The Big Bang Theory (Large Hadron Collider; do I need to say more?) and Boston Legal and Two and a Half Men in the pure comedy side.
I also liked the time back when CN's cartoon were actually cartoons and good at it. Samurai Jack, Dexter's Laboratory (+ "Dial M for Monkey"), Cow and Chicken, and the Original Powerpuff Girls (darn fairy!!!).
On the documentary side, definitively Mythbusters and Planet Earth.