I'd have to say...
Miles Davis - Bitches Brew
I don't know what to say about it. I'm sure you already know.
Prince - Sign O' The Times
Already discussed well enough. Prince is my musical hero.
2Pac - All Eyez On Me
Maybe not as epic as some others, and maybe not every track is a classic, but the whole thug life gangster rap thing is snugly encapsulated into one sprawling masterpiece (and I would call it a masterpiece). Plus this is just my top three, and I could listen to this all day.
I almost said 69 Love Songs by The Magnetic Fields (a triple, I guess that's ok), but I couldn't take off any of the three I said. It is great though. Probably well known around here, but a lot of people aren't familiar with it, so if you haven't heard it, you should.