but when I listen to a 30 second sample of a song I already know I always think, "geez they picked the wrong part of the song..." I would overlook a lot of potentially good stuff if I did it your way, personally.
Sandwich makes good points: 30 seconds of a song is often not even enough to tell what a song is really like, and B_M's way is wrong 
There has NEVER been a single album made where I like every song on it (other than the occasional Greatest Hits album, which is just a compilation of various songs from a bunch of albums)
This makes me sad. "perfect album" is one of my last.fm tags. There aren't many, but they exist for me.
I guess if you held a gun to my head and said PICK ONE, I'd have to pick Dark Side of the Moon. It's the only album off hand that I can think of that I actually can listen to without actively hating at least one song.
I've never heard a "Greatest Hits" album that didn't have at least one popular piece of shite in it that you've already heard one too many times.
DSotM is a perennial, and without it, as Carrotflowers points out, there would be no universal speaker test.
Some other contenders: the run of Stones albums from "Beggars Banquet" to "Exile on Main Street," the Velvet Underground and Nico (of which it has been said 200 people bought it and all started bands,) the first three Elvis Costello albums, the Clash's "London Calling", Miles Davis' "Kind of Blue," Coltrane's "A Love Supreme."...
David Sandborn, a very successful and influential commercial sax player, once told me in an interview that the 20-something minutes of an LP side was the perfect length for sustained artistic expression. Much more than that and people get distracted. It was much harder after the dawn of CDs... too much capacity, so outtakes and weakest links that should have been left off. It's hard to get the originals without a few, often substandard "extras.'
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