I love sit/coms, so to choose only one is difficult. Mary Tyler Moore show however was excellent. The whole cast was so much fun. I found it on You-Tube, but no word embeded came up. If anyone else can show a clip to us, that will be fine with me. It was a funny funny show.
Also, Fraiser has been excellent as well as Seinfeld. So very many really good ones. You can't beat I Love Lucy and of course Mash. But unless it's really dated by now, and it probably is, The Mary Tyler Moore Show was wonderful.
Oh, I forgot, Murphy Brown. That was great also. There have been so many wonderful situation comedies. Thank goodness!