I'm going to keep it to one album per band, even though several bands on this list could have multiple entries.
The Flaming Lips - The Soft Bulliten - A really beautiful album with no songs worth skipping. My favorites on it are probably "Waiting For A Superman" which has a great piano part, and "A Spoonful Weighs A Ton"
Pink Floyd - Dark Side Of The Moon - A very popular choice it seems, but there's a reason for it. This album flows together so well, every time I listen to it, I make my best effort to listen to the entire album in one sitting, it's the only way to do it justice.
The Beatles - Sgt. Peppers Lonely Hearts Club Band
Led Zeppelin - IV
The Mars Volta - De-Loused In The Comatorium - A masterpiece, every time I listen to this album I hear something new that I never heard before.
Radiohead - The Bends - More acoustic than most of the band's other work, but I believe it is their best album. Every song is great, but "Fake Plastic Trees" always blows my mind, the end of the song just has so much emotion in it.
Red Hot Chili Peppers - Blood Sugar Sex Magik - Tough choice between this album and "One Hot Minute", but I think this one takes the cake. A really fun album to listen to.
David Bowie - The Rise And Fall Of Ziggy Stardust
The Pharcyde - Bizarre Ride II - One of the best hip-hop albums I've ever heard, a talented group who don't take themselves too seriously.