My all-time favorite sitcom would have to be Scrubs. I also think The Simpsons nowadays is hugely underrated. Don't know why the newer episodes get so much hate, I've enjoyed every season thus far. It's a pleasantly funny show with character at its core. Just because it doesn't make its audiences guffaw like idiotic middle schoolers hearing their first sex joke, doesn't mean it's not good. Blows my mind that some people find South Park more enjoyable than The Simpsons.
But seriously, Scrubs rules. There's a charming quality to it that makes it the ideal buddy show. I have a couple friends who, like me, have seen every episode. During many conversations, one of us will reference some obscure line and the other actually remembers it. It's that kind of show. Everything (and I do mean everything) in it is quotable if you're with a best friend.