The big 2 for me are.
A Tribe Called Quest - The Low End Theory
I know every song on this. I have this on heavy rotation in my car and have played the hell out of it.
Radiohead - Kid A
This has followed me/haunted me through high school to present day. Know every song on here and have heard it played countless times. Always come back to it for some reason.
Honorable Mentions
Aphex Twin - Richard D. James Album
The Beach Boys - Pet Sounds
Leonard Cohen - Songs of Love and Hate
Pink Floyd - Pipers At the Gates of Dawn
Pink Floyd - Dark Side of the Moon
Pink Floyd - Wish You Were Here
Slint - Spiderland