I saw my first Dead show in the summer of '78 after my freshman year in college, got hooked, and saw close to 100 shows between then and '95. Saw some great shows, saw some weak shows, and everything in between. 1991 was the only year I didn't catch at least one, and I think the most I saw in one year was five.
A couple of memorable moments:
Flora Purim and Airto guesting on a particularly spacey "Space" at Long Beach in 1980 - Flora was was doing some really freaky wailing.
Eugene in '93 when during the Jack Straw opener Bob Weir's guitar malfunctioned, and he walked off stage for awhile leaving the rest of the band to jam away on what must surely have been the longest ever version of that song. (There is a nice matrix of this show floating around - it also has a really long 2nd set).
Chula Vista in '85 when they started the 2nd set with a little Space thing with some Twilight Zone teases thrown in (they had just done the music for the credits of the 80's version of the TV show) and then right into one of *my* favorite songs, Scarlet Begonias.
L.A. in February '89 when Bob Dylan sat in on the 2nd Set. He seemed to not want to sing, but Weir pretended to "forget" some of the lyrics in Memphis Blues which coaxed Dylan into jumping in.
And of course there are the amazing versions of so many songs over that time frame. Or the pleasant surprise of hearing a song from one of the shows I was at appear on a CD (the Bird Song from Without a Net was from an L.A. Forum show in December '89 that I was at, and the She Belongs to Me from Postcards of the Hanging was from the '85 Chula Vista show).