As you'll know if you've read my House M.D. topic, that's my favorite TV show. XD For various reasons, but mainly because there are so many good actors and so much good character development on the show.
Other shows I currently watch include Lost and 24, though I wouldn't call them my favorites. Lost was brilliant for the first two seasons, but apart from a handful of episodes, season three sucked. It lacked the plot continuity, character development and stellar acting that marked s1/s2, and the idea that the writers might not be able to explain everything that's happened on the island is lurking in the back of my mind whenever I watch Lost now, because it's just so convoluted. And I haaaated the s3 finale.
I've never had faith in 24 as far as its plot continuity, so I'm not frequently disappointed by that show--since I have no expectations in the first place.
My top two favorite characters were both killed off by season three, and honestly, season one was the only *really* good season. I'm not sure if I'll watch season seven, since I think I might have a better use for those 24 hours of my year.
However, there is one more TV show that I absolutely adore, almost as much as House, though it's been cancelled now: Firefly. Only fourteen episodes were made, and only twelve of those aired, but I have the DVD set and I've watched them all several times. Even the worst episodes are better than a lot of supposedly "good" TV nowadays; I think that if Firefly had lasted for a few seasons it would have been spectacular. The universe is so well-constructed, the characters are so interesting, and the actors all work superbly together as an ensemble--there are nine main characters, and without the right group of actors that could be a disaster. With Firefly it wasn't.
So I guess if I had to choose my top three favorite TV shows of all time, they would be House, Firefly and Lost, in that order--though there's a fairly large gap between #2 and #3. *sigh* Ah well, we'll see how s4 of Lost turns out.
(And wow, I seem to have been making a lot of really long posts lately... XD)