This is definitely a good idea... lots of things to check out if they ever show up as reruns.
I'm surprised no one listed the Dick van Dyke show... it was one of the originals. Except for occasional sports I didn't watch TV until I had kids so DvD was in reruns before I saw it. My kids were 10 years apart so I watched them all twice. (I was subjected to Gilligan's Island and I Dream of Jeanie twice, too. Yuck!)
Mary Tyler Moore was the only sitcom I watched when it aired - my wife and I never planned anything for Saturday night because we couldn't bear to miss MTM.
Two of my other favorites that got no mention are 'House' and 'Monk'... maybe they don't qualify as sitcoms. I really liked the combination of humor and a strong story line.
"Soap' was one of the most enjoyed shows in my house - but not by me. My wife's mom used to come over to watch it with her and two would be laughing so hard I couldn't hear half the dialog.
'Fawlty Towers' was great. I enjoy Brit humor... 'Are You Being Served'; 'Yes, Minister', 'To the Manor Born', even Bennie Hill, up to a point. 'Last of the Summer Wine', 'As Time Goes By'.
I sporadically enjoyed most of the old standards: Seinfeld, MASH, Frasier, Cheers, Bob Newhart, WKRP, All in the Family, etc.