Five essential progressive rock/metal albums:
1. In the Court of the Crimson King-King Crimson. The album that pretty much started the wave of prog in the 70s and late 60s. Zappa and Sgt. Pepper shook things up, King Crimson took that sort of experimentation and made it work (for the most part) on this album.
2. Dream Theater-Images & Words. If not for the popularity of "Pull Me Under," I doubt prog's 90s revival would have had the same impact. Porcupine Tree, Spock's Beard, and the Flower Kings all got record deals around 1992, and Dream Theater showed that prog could indeed become popular once more. Now we have Mastodon, Coheed & Cambria, Muse, and so many others in the "mainstream" carrying the prog torch.
3. Pink Floyd-Dark Side of the Moon. There are two albums everyone owns. One is Frampton Comes Alive, the other is Dark Side of the Moon. This album cemented prog's popularity until disco came around, as well as opened doors to thousands of new fans willing to see beyond one incredible album.
4. Genesis-Selling England By The Pound. Before Phil Collins started destroying rock music in the 80s, Genesis was led by Peter Gabriel, one of the few people the term "musical genius" could actually apply too. After a few albums with the now-classic 5-man lineup, the band found all of their strengths in their masterpiece of the Gabriel-era. There are countless "new-prog" bands that try and imitate the sounds you find on this album, but they just do not come close at all.
5. The Mars Volta-De-Loused in the Comatorium. A tough choice. I knew I had to have one album from this decade, which has only continued the 90s prog revival, but there were so many bands and albums to choose from. Ultimately, I chose this one. My favorite Volta album and while sometimes a bit too busy for my tastes, this band went out and did something very different from most other rock bands in the early 2000s. Latin grooves, Crimson-like improv, mixed with trance and electronica, and you have one monster of an album.
@ mberan: Nice list! Though I'd personally pick VH's self-titled over 1984 and put Born to Run by Springsteen in place of CSN&Y (still a great album though!). I think I might post my own classic rock list later.