I'm hesitant to answer the TV question, as I'm likely to participate in that thread whenever it comes up. Also, because I am a lifelong TV addict, I find it nearly impossible to separate nostalgic fondness and guilty pleasures from my favorites and look at some objective idea of "best." That being said, "The Wire," "Firefly," "Homicide: Life on the Streets," and "The Simpsons" are all in the conversation.
As for movies... my top five is as follows:
New York, New York
Down by Law
A Streetcar Named Desire
Dead Man
Pulp Fiction
The list of films fighting for that 6th place is entirely too long to post, however.
Also, like Slim, I think there are different levels of "best." I consider the above five films to be the best I've seen in terms of technical film making, acting, writing, etc., as well as in terms of engaging and entertaining me and, therefore, consider them among my favorites. However, with the exception of Pulp Fiction, none of them are particularly easy to watch, so I tend not to pop them in the DVD player as often as something with lots of laughs and/or 'splosions. For example, if I were to think of my favorite movies, the list would probably include Point Break, though that wouldn't be anywhere near my list of best movies.