If nothing else, Seinfeld was definitely the most revolutionary SITCOM of all time. Those first few years (and even during the peak years) it truly could be a show about nothing. My all-time favorites are the 'Parking Deck' episode and the 'Marine Biologist' episode.
MASH is tougher to put your finger on since it changed so much over the years. It started out a goofy, light-hearted comedy and eventually evolved into a moralizing comedy/drama. I personally love the show, but sometimes flip away if it's a serious episode. I like to watch sitcoms to relax, not get depressed.
Like a lot of people, I personally think Cheers is probably the best sitcom ever. It wasn't revolutionary like Seinfeld and hardly ever made me laugh anywhere near as hard as I did at portions of Seinfeld. But for what it did, it did better than anything else in my opinion. It had great writing, great actors, great characters, great stories (before the Seinfeld revolution of no stories, just funny conversation) and a great atmosphere. No other show helps me relax as much as Cheers. I guess that's why it's so popular and successful in late night syndication. Hopefully it'll always be that way.
Perhaps the coolest thing about Cheers is that it seems like nearly every generation likes it. People in my grandparents generation (the WWII generation) seem to like it. People in my parent's generation (the Baby Boomers) seem to like it. People in my generation (GenX . . . I hate that title) seem to like it. Even kids the age of my nieces and nephews seem to like it. It has that broad-based appeal that just clicks for some reason. I realize not EVERYBODY likes it, but it's mature enough to be funny for adults but isn't so offensive with in-your-face sexual innuendo that your kids can't watch it.
