I like my Dead really jazzy. I want piles of exploration and improvisation. So I love i prefer the 1971 - 1979 era, but IMO the Dead reached their peak in 1973. There are some truly some awe inspiring shows in that year. My favorite is probably 6-10-73 RFK Stadium. It is a total monster. The Bird Song, Playing, Eyes > Stella segment is particularly great. A truly amazing Dark Star that runs 26 and 1/2 minutes in the second set. Followed by a third set packed full of bluesy tunes with Dickey Betts sitting in.
I'm not a big fan of the Dead past about 1979 and honestly haven't listened to very many shows from then on, but I really dig the shows with Branford Marsalis sits in particularly the new years eve 1990 show. To me he brings out the jazz flavor of the dead that I really dig.