It's really hard for me to narrow down a favorite television show. My favorite show could change from day to day depending on what style of show I'm interested in watching. While the topic is "What is your favorite TV show ever", I'm going to cop out and list my favorites in their respective genres.
Drama - The Wire
While this show slightly faltered in its fifth season, the show taken in its entirety is probably one of the finest examples of what TV has to offer. It can also be one of the hardest shows to watch, especially the ending arc of season 4.
Freaks and Geeks
Comedy - Arrested Development
In my humble opinion, this is the best comedy to air on network television. Hyperbole, perhaps. It appeals to my particular sense of humor (clever word play, meta, and slapstick).
Curb Your Enthusiasm
Sci-Fi/Horror/Adventure - Quantum Leap
This is one of those things that may change tomorrow. Today, I'm in a Quantum Leap mood. Truth be told, I'm not a huge fan of this genre. I can really only count the shows that I've seen in their entirety or seen the most. Quantum Leap meets the later criteria. I'm sure once I finish Battlestar, Buffy or Angel one might take over the top slot, but Leap just hits that right bit of nostalgia for me.