Whoa!!! Now that's a difficult question, particularly since many of us relate to hearing/seeing/experiencing The Dead live moreso that their studio albums. I don't consider myself a Deadhead - I've only seen them in concert roughly 20 or so times, rarely sober and mostly, well... you know - but if ever there was band whose studio efforts paled in comparison to the real deal, The Dead would be it.
Having stated this, I am partial to American Beauty as an album, but, and a big BUT, side 2 of Terrapin Station is, well, think of what Echoes is to Floyd's Meddle. I still remember where I was, the girl I was with, and my reaction to that song, the first time I heard it, 30 something years ago.
Thanks. I've been a paralyzed by the sheer volume of available material. Gonna use the suggestions here as a starting point.