I find it hard to compare cartoons to comedies to dramas to whatever else. I also find it hard to compare the emotion I felt over a TV show ten years ago to one I watch now, because I can't remember things 100%. I also don't watch as many different TV shows as some other people do.

Still, I'll give it a go!

When I was a kid, Transformers was the best thing ever. Simpsons was awesome, too.
When I was a teenager, Friends was the best show ever, and I never missed an episode. I think it remains the only show that I have always watched, even in it's later years (not counting shows that are still running right now). Star Trek: Voyager and Stargate SG-1 were cool, too. Oh, and Roswell.
Over the past ten or so years, I really enjoyed shows like Lost and various stand up comedy shows. I'm not sure if the latter count...
Now, though, my favourite show is Suits.

In the end, it's probably between Friends and Suits, and since Suits is still going I suppose I'll say my favourite ever complete TV series is Friends.