In no particular order...as they come to me...
The Beatles - Rubber Soul --- This is the first album where the Beatles stop writing pure love songs and start to experiment, both musically and with drugs. A great transition album
The Beatles - Abbey Road --- Pretty self explanatory. IMO the culmination of their creative and musical efforts.
The Rolling Stones - Exile on Main Street --- For what rock n' roll sounds like
Pink Floyd - Dark Side of the Moon --- Pretty self explanatory
Jimi Hendrix - Are You Experienced --- One of the defining sounds of the 60s
The Clash - London Calling --- First wave of punk in Britain, as well as the forerunner for post-punk, and later, postmodernism in music
Guns N' Roses - Appetite for Destruction --- This is why rock is so dangerous and powerful
Van Halen - Van Halen --- A straight forward rocker that revolutionized the way guitars were played
Iron Maiden - Number of the Beast --- Just a great example of early British metal, and a great gateway into metal
Nirvana - Nevermind --- I hate this album with a passion and everything it stands for, but if you're looking to compile a list of albums that define a generation, this is it for the 90s. Almost every album within rock post-1991 has been affected by this album, however indirectly. Also, IMO, destroyed rock as the 70s and 80s knew it.
Those are my choices...I don't know if I'd stick with them tomorrow, heh...they came to me off the top of my head.