Nothing but Soundgarden, in chronological order of major release. All three are very complete and deceptively wide-ranging; Badmotorfinger is underrated and Down on the Upside gets too much attention. Of the bands most people have heard of from the early-to-mid 1990s, is there a more under-appreciated group? Nirvana, Pearl Jam, Alice in Chains and the Smashing Pumpkins all get more respect; from top to bottom, all inclusive, I'd say only Alice in Chains (and perhaps Nirvana) could rival Soundgarden's ability to put out LPs full of solid songs. (Album-making, as separate from albums full of good songs, was never a strength of alternative music.)
If you're on iTunes, pick up the pair of "Songs from the Superunknown," and don't forget "Birth Ritual" from the Singles soundtrack either.
Dana, where does Soundgarden fit in your musical hierarchy?
Addendum: From Badmotorfinger, enjoy especially the second half of the album; the rolling "Searching with My Good Eye Closed," the saxophone (in grunge music?!) highlighted "Room A Thousand Years Wide" and the calculated "Mind Riot". "Holy Water" is a look ahead to Superunknown.
From Superunknown, focus also on the rear half of the album-- "The Day I Tried To Live," "Fresh Tendrils," "4th of July" and "Like Suicide" are all top notch songs that trend away from the mid-90s radio friendly.
Down on the Upside is my least favorite, personally, but still shines in areas. I do think the radio-friendly tracks from this one are the best, especially "Blow Up The Outside World," though Thayil's at his best in "Tighter & Tighter" and "Overfloater" is subtle enough to enjoy three or four times in a row.