Hmmm... tough...
Currently:
1. The Simpsons
For the first twenty or so seasons, at least. But even the worst of the Simpsons is still watchable.
2. Scrubs
Banana Hammock! Banana Hammock! Banana Hammock!
The second series with the new cast, not so much.
3. Firefly
Too bad it only lasted one season. Thankfully, we have the comics. "Those Left Behind" is one of the few new comics I actually purchased within the last five years.
4. Avatar: The Legend of Aang
It really only starts picking up near the end of Season 1, but it's beautifully made, has some of the best and most realistic Kung Fu ever in a TV series or anime, and it's both approachable enough for kids to watch and deep enough for their parents to appreciate. The closest any kid's cartoon series has come to perfect.
5. Mythbusters
Jaime: "So... we didn't get the results we wanted. Myth busted."
Adam: "I still want to see an explosion!"
Jaime: "You know what THAT means." (rolls out chest full of C4...)
6. Top Gear UK
As journalism and as truth in journalism, it's down near the bottom of the barrel, but damn if it isn't entertaining watching them destroy things.
7. Iron Chef (Japan)
Fight!
8. The Angry Beavers
Did not. Did too. Did not! Did too!
9. House M.D.*
Aside from the fact that all the female doctors on the show are impossibly hot, bar the one they killed off for not being hot enough, fun show to watch.
10. Being Human (UK)*
Slow in parts, nonsensical in others, but somehow compellingly watchable. Too bad they dropped the ghost girl from the pilot episode, but her accent was so thick I don't think anyone could understand her dialogue without subtitles...
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*-might change mind in the future... but the top 8 are solid.