Avatar: The Last Airbender, because it managed to overcome the juvenile pile of crap that is children's television to become not only a good children's show, but one of the best shows ever made and one of the greatest fantasy stories ever told. Truly amazing series in every regard.
Firefly, because Firefly. I would like to think I don't even need to explain why this show was amazing, though it would have been far better had it been given time to properly wrap itself up. Fox is evil.
24, because it was a great idea and was pulled off season after season with incredible finesse and originality.
Clannad, because of its ability to reduce even the most hardcore person to a blubbering mess. Seriously, if you didn't cry in that show, you don't have a soul.
Also, though I wouldn't put it up in the absolute best yet, Wakfu is amazing. If you haven't seen it, do. Really, really good.
EDIT:
Saltyk said:
People have stated a lot of great shows like Scrubs, House, and many, many others. But I didn't see anyone mention Avatar: The Last Airbender. Have you all forgotten that great show?
I didn't! 'Cause it's the greatest show ever made. True story.