James Brown - Star Time (4 CDs)
Because although you *could* own James Brown albums, this set (plus Live at the Apollo) is almost everything that anybody except a fanatic would want or need, and virtually everybody who is a music fan wants or needs James Brown music.
Jethro Tull - 20 Years of Jethro Tull (3 CDs)
The opposite of Star Time - this set was for nobody other than fans of the band in question. But in 1988, and for a good 15 or so years thereafter, this set was a godsend. There was a bunch of live material, which was great because the band had only one live album to their name, at that point. There were BBC sessions. There was the Chateau D'Isaster tracks. There were B-sides and never-before-released studio tracks, and most of them were pretty good. It came with a booklet that had a ton of information and pictures that hadn't previously been available (no books about Jethro Tull existed in 1988). Granted, the sequencing (apart from the first 6 tracks) was lousy and there inexplicably was 7 or 8 songs on the set that already had been released on the albums (even more inexplicably, only one of those - Bungle in the Jungle - was any kind of a hit). Until this material leaked onto CD reissues of the regular albums, this set was a must.
Neil Young - Decade (3 LPs).
For a long time, the gold standard of the pre-CD era.
Ray Charles - Birth of Soul (3 CDs)
It was by far the most practical way to get this material, which is great.