While I will agree that Seinfeld was a very influential show, and many do consider it one of the better all-time sitcoms, I would say this is also biased by your age. I would say that The Dick Van Dyke Show, The Honeymooners, and All in the Family set the standards and made modern sitcoms possible.
As far as "Greatest Sitcom", of course that's opinion and everyone's will be different. I'd be hard pressed to decide my all time favorite, but my top 5 would be MASH, All in the Family, Cheers, Seinfeld, and The Simpsons(1st 10 years at least).
As for Two and a half Men, it's not a "Great" show to me, but it is funny, so I still watch it.
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