Led Zeppelin IV.
In fact most of the Zeppelin catalog sounds timeless, which IMO comes down to two things: excellent songwriting and musicianship, and a significantly less processed approach than today's standards. Despite the audiophile's insistence that recording analog, as opposed to digital, was the key, I think it has less to do with the recording medium than a stronger focus on tracking as opposed to slapping a bunch of unnecessary garbage on the tracks during mixing.
Same with the Beatles, their stuff sounds fresh today because they didnt have much, or need much, to record their music. What you hear, particularly on the earlier albums, is four musicians doing everything in a single take plus a few vocal overdubs. Much of this was possible due to their slogging it out 24/7 in the Hamburg clubs.
As to current music, I don't know whether Tool's music will sound good twenty years from now, but Aenima still sounds amazing after ten, and I suspect it has more to do with their excellent writing and talent than anything else.