Holy shit, this is tough. I think I'll have to go with Hot Fuss by The Killers. I have used and abused that album over the past five years. Kind of mainstream but an excellent album.
Other contenders were Yoshimi Battles the Pink Robots by The Flaming Lips, Give Up by The Postal Service, Up The Bracket by The Libertines, and Good News for People Who Like Bad News by Modest Mouse.

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