Some great suggestions here, but I have to add my all-time favorite (so far, at least):
4/8/72 - released on "Steppin' Out With The Grateful Dead"
This version is such a masterpiece! It's about 30 minutes, and explores a whole lot of territories. In the beginning everything is nice and melodic, and they build up some tension before the release of the first verse somewhere around 10 minutes into it. Then, not long after the first verse, everything collapses into a dark, moody, emotional, chaotic space - but as they play on they all struggle to get out of this and while they do so they try many alternatives, until, in the end (about 26-27 minutes into the song) (they skip the second verse as there's obviously more important things to do), they just build it all up to a huge wall of tension and sound - and out of this chaos comes Garcia like a golden Phoenix, reborn out of the chaos, and starts playing the simplest, yet most beutiful melody I've ever heard! The rest of the band jumps onto their savior and joins him immediately in this joyful hymn-like song which sparkles with life and joy and happiness! This goes on for a minute or so, and then they move into a Mind Left Body-ish jam, and if you listen carefully, you can hear/feel "Sugar Magnolia" lurking in the background there. Then, all of a sudden, she (sugar magnolia) comes sliding in like a red hot knife through butter, as if to continue celebrating the new-found happiness.
All in all the interplay on this version is really great - each member obviously listens carefully to all the others, and takes the lead or follows the leader, depending on what's going on - and the musical themes explored are a times amazing.
I listened to this one on my way to school today, and thought about writing something about it, and then I found this here thread now...
I think there's a SBD on gdlive.com if you don't have Steppin' Out. Give it a listen, you won't regret.