I see it as 4 distinct variations of the band:
66-70 - Early Psych, Bluesy rock, epic, primal Dead (Dark Star, Other One, St. Stephen)
71-74 - Strong songwriting, Cowboy songs, really solid output defines what we know as Grateful Dead. Jerry singing and playing at his peak. Mickey dropping out for the period, in my opinion, really allowed the nuances of the compositions come forward, Billy really cooks on the jams. Solo works and studio albums (Wake, Mars, Allah) are amazing. (Playing in the Band, Eyes of the World, 1/2 Step, Loser ... too many to name!)
75-79 - Taking a bit of a break form live shows really helps the band create a new focus, I think. once they started playing shows again there was a new fire under their playing, Mickey comes back, new songs are fast and funky, set style really takes shape. 1976 shows at smaller theaters were amazing. Studio albums well received. (Shakedown Street, Sampson, Terrapin, Estimated Prophet)
1980-1995 - Many will argue that there are two waves here, Brent era and post Brent, but really the band didn't change sound all that much, or really its just that the 90's Dead were not that great (sorry!). Once Brent died they just never got back on track. 80s were pretty ripping, especially the early years. But the 90 were not that great, (new box set of Spring 1990 has some great stuff, but not enough IMHO). I did a bunch of shows in 1985, Dylan tour and spring in 1986 and did the 1987 summer tour (Telluride!), Super fun, great times, but not the strongest musically.
If I had to pick the best 10 year span of the Dead, it would be 1969-1979 hands down