Oh dear. I'm not sure I can answer this. I'm so very fond of the casts of so many shows. Because I'm so character-oriented, I tend to like or dislike a show based on its characters and casting. If I find the characters miscast, I don't like the show much. And how do I decide which cast is best? Best actors? Best-looking? Best mix of different types? I don't know.
Maybe I'll just go with one of the biggest TV show casts ever: Lost. They're all excellently cast -- I can't think of a single character I'd rather see a different actor play. They're an incredible mix of talented people, and while they may not be my favorite cast, they're definitely awesome.