Andrew: That's a hard one, as the Dead produced a lot of records and are famous for a live repertoir of unequalled breadth (many songs and covers that never made it to a record). Others may disagree or have other suggestions, but I would start with the Europe '72 collection (3 record live set). The music is an eclectic mix of R&B/Blues, Country and Rock. It includes some some excellent performances (check out Tennessee Jed, One More Saturday Night, He's Gone, Jack Straw, for starters).
But apart from this, it showcases Phil Lesh's Almembic-modified Guild Starfire (see the link above for picture). Clearly, though this bass was a predecessor to the first entirely Alembic-built basses, it had a very cool and very identifiably Alembic sound. Suffice it to say, Phil is a unique player. Hope that helps.