I'll pick one at a time, it's really tough to choose in order, there are more than a few I'm very fond of...
....however, to be true to the thread, overall, I'd pick this as #2...#1 I'll save for a little later.
One not mentioned yet is the cast album of HAIR, that huge 1968 success that spawned a few other chart albums (like "DisinHAIRited"), and many that didn't including very interesting variants from France (on Philips Lp, 1970) and Japan (RCA, 1971). THIS one, of course, was actually the second go at the show, since the first version (which was off-broadway) was issued in late 1967 (also on RCA Victor). Between then and the million-seller, a few songs were dropped, others moved around, and everything, of course, rerecorded.
What I like about the CD-4 is that the mix is very playful: instruments and sounds move around the room, and because there are many and varied voices being heard (singing or not), the remix to quad puts them in every corner, here 'n' there, a sensible use of panning and circling that only occasionally sounds gimmicky. Of course you know the big ones: "Aquarius," "Easy To Be Hard," "Be In (Hare Krishna)," "Good Morning Starshine" (my wife's theme song, FWIW
) "Let The Sunshine In," "Where Do I Go" and the title track all made the U.S. charts by somebody (a few others not mentioned did as well, although it took about six months after the album's release for artists to cover the material with regularity (by that time the album was #1). Also, the instruments are often very discrete--the singing is really more elaborate at times than Galt Mac Dermot's arrangements, which work just right but rarely overwhelm everything. Think 'simple and to the point.'
This is the kind of time capsule that's a guilty pleasure if you were there (even if you liked it as much as I did!), although now a bit obvious and stodgy, a strange product of its time. But the quad mix would, I think, impress anyone who digs multichannel music.
ED