Lonson is right - there are a lot of different Grateful Deads. IMHO the best is Spring -Summer 1977. 1978 is really good too as is 1976. But something very special happened in 77 that never was quite captured again. 1972 is really good period too. Listen to all the Europe 72 shows. They’re great, but they lacked the deeper song catalog they had in 77. Which makes 1989-91 really interesting too. By 89-91 their song list was very deep. Where in 72 the shows had about a 50% repeat from night to night, by 1990 you might get a repeat once a week. But even in the '72 the repeats were always unique, same song but played very differently.
Thing is every tour (Spring, Summer, Falls in any given year) they had a different sound. It’s fun to experiment and find the sound that suits you, but do it with the different years in mind. Listen to some '72s, 74, 76, a lot of 77, some 78, carefully select your 80s listening (the lost decade) and then listen to 1990.