Metallica's Black album. And not because the music was the greatest, the band was arguably past their prime.

But this was the tape (yes, cassette tape) that I hiked many miles through Yosemite to, over passes, through canyons, down the side of waterfalls. It may have been the only one I had bothered to pack, but this tape gave me something to march on to.

~30 years later, and it is immediately my first choice, although I know far better albums, musicians, etc.