Led Zeppelin — Houses of the Holy
I think this is absolutely their most complete and perfect studio album. The songs have a huge range, from beautiful and soft on The Rain Song; to the loud and popped out Dancing Days; to the dark, riff-driven proto-metal of No Quarter. Also, the riffs are great. The Ocean is still *the* classic rock riff in my opinion, and I get a huge kick out of the James Brown tribute that is The Crunge. Fantastic album, all the way through.
Yes — Close to the Edge
I could talk for hours about this album, but I'm being merciful, so I won't. Basically, this album is extremely ambitious, beautiful, fantastically complex, and technically proficient. Unlike a lot of prog rock, old and new (I'm looking at you, D, Yes' technical ability and complicated arrangements don't inhibit the beauty and harmony of their music. If you dig classic rock, prog rock, or just great music in general, you need to have this album.
Mahavishnu Orchestra — Birds of Fire
Totally instrumental, basically just 5 ludicrously talented jazz musicians getting together to play some kickass jazz-rock. Birds of Fire is tragically lacking their best song (Meeting of the Spirits imo), but it's made up for by other standouts like One Word, Miles Beyond, and Open Country Joy. Billy Cobham and John McLaughlin are absolute monsters on their instruments, some of the best all-time.
Mk, think I'm done for now. Oh, and I'll toss my vote in for Sgt. Pepper's as the Beatles album.