Recently, I've really enjoyed the "Xanadu Master Edition Series" from Elemental.
I also have quite a few 32Jazz reissues of the Muse catalog. I'm not sure whether those count as a "series." But, in any case, it's some of my favorite music.
For vinyl, I really like "RCA Victor Vintage Series" -- most of them had the wine-rack design on the cover. Between Bechet, JRM, Duke, Fats, and others, there's just so much great music there. ...And I still enjoy spinning those LPs -- even if most of the music has been reissued more comprehensively in other sets.
There's a lot to be said for an artfully-programmed single LP of music originally released as 78s. Often, they feel more approachable than the huge, "impossible-to-hear-in-one-sitting" boxes.*
* Not that I'd ever want to give up any of those box sets!
Oh yeah. Good call.