For me it's MASH.

The biggest reason for me is how thoroughly and brilliantly they explored its setting and what a TV dramedy could be. Smartly written comedic episodes, very poignant dramatic ones, a bunch of episodes that blurred that line, and quite a few experimental episodes that pushed the format of the series in very creative ways.

And for me, it has my favorite cast on a TV series. Through the emotional highs and lows, you get to know these characters so thoroughly that even some of the bigger caricatures have their moments of humanity (even if it's more hidden in someone like Frank). And while some actors are stronger than others, I thought they were all very good in their respective roles, and did a very admirable job playing the various emotional beats they had to throughout the seasons and bring their characters to life as well as they did.

Edit: I will say that Lost used to be mine as well, and while that's no longer the case, it's still pretty high on my list.