Man, it is an incredibly difficult task to pick what I consider the best shows.
Cartoon would be between King of the Hill, Daria, Invader Zim, and Simpsons. If you take the best of Simpsons, it almost trumps everything out there, but the same could be said for King of the Hill. Daria and Invader Zim are much more concentrated, and thus longevity has to be taken into account. Honorable mentions go to Futurama, The Maxx, Spongebob Squarepants, and the Critic.
Sitcoms are between Seinfeld, The Office (American), and the Wonder Years, assuming Wonder Years counts as a sitcom (since it really isn't one).
I don't really watch anything else, as I'm not a fan of drama (it normally comes off as cheesy to me), reality tv (ugh), or gameshows. In this nebulous category, I will say that Alton Brown kicks a motherload of ass, and in the realm of cooking shows reigns supreme.
If I had to pick one to watch forever, thereby making it the best, it might have to go to Simpsons. There's a good 6-7 seasons of pure genius, and I can't say the same for every other show. But even then, I think I could say the same for Invader Zim, as it edges out things like King of the Hill with some amazing animation and some brilliant characters.
Very difficult question. I'm probably forgetting some shows also.