The best Michael Jackson collection of his Motown solo days can be found on
Gold, which I just purchased a few months ago. The mega-fan could go for the 4CD box set
Ultimate Collection which goes from the Jackson 5 days to his Epic solo stuff.
But his maturity and greatest solo work didn't begin until
Off The Wall in 1979 which some believe is a better album than the followup
Thriller. I still prefer
Thriller's fun of classic singles and the maturity that it shows, but make no mistake,
Off The Wall is his best pure R&B album.
After those 2 classics there's a letdown with
Bad, but it manages to be a decent followup.
Dangerous and his last album
Invincible get lost in the shuffle, but dig deep and you'll hear some songs that stand up to his canon.
The best collections of his solo stuff are
Essential and
Number Ones. But now that he's gone you'll likely see more, and frankly I'd like to see one on his non-hits, both from Motown and Epic.