I think its all relative. Seinfeld is a great show. But i think with comedy shows you have to go by generations/era's. For example, I don't find the original Three Stooges very funny. But I know back in the 40's or whenever it was on, it was the most hilarious thing ever and people who grew up with that still think so. Seinfeld is the best comedy of the 90's and one of my favorites, I just think its impossible to do "greatest shows ever", we can easily do "favorite shows ever". In 50-60 years, I'm betting Seinfeld will lose its humor. Right now, IMO, The Office is the best comedy of the 2000's and I think its funnier than Seinfeld.
"Let's Keep The Friendly Confines Friendly" - Ernie Banks