Obviously, this is a highly subjective question. However, in my opinion, the following stand out from the crowd:
All In The Family
M*A*S*H
Seinfeld
Each of these were very strong on the following three elements
Well written
Incredible cast
Pushed the envelope by touching on subjects that were considered very controversial for their time
There are plenty of great shows that touch on one or two of these, but not all three. I am a huge fan of The Office, but I don't really consider its content to be "groundbreaking". Personally, I never thought that much of Friends. Characters never really clicked and I thought most of the writing was predictable. But, again, this is a highly subjective question.