Some terrific options to choose from.
I went with All In The Family because I think it was the most cutting edge for its time. Two and a Half Men would never be able to get away with half of what it does had it not been for ground breaking efforts of earlier shows.
There is clearly a generational difference as you look through the list and some of the other shows that have been mentioned. It makes it tough to compare across the years as the rules and boundaries have changed. Even MASH went to places that were rarely visited before.
From a pure comedy point of view, I'd also consider The Beverly Hillbillies and Get Smart. Bear in mind, I was a kid then and the catered to my demographic, but I sure did laugh.
Seinfeld and Friends were both huge hits through the years, but again, targeting a very specific demographic.
Cheers was great and had one of the best spinoffs ever with Frasier. I'm sure some would pick Frasier over Cheers.
The show is currently at the top of my list is The Big Bang Theory. Again, it works in this day and age, but it does manage to please a very wide demographic.