A great box set for me is one that makes you consider an artist whose work you know well from a completely fresh perspective or one which presents new material in a genre that is new to you and makes you re-consider your old assumptions.
Examples of the former would be the Mingus Complete Debut box and Mosaic Dean Benedetti box.
Mosaic are particularly good with the latter - the various Ellington and Armstrong boxes, Teagarden, Stuff Smith etc. to name a few.
Then there are also Rumsfeld's 'known knowns' and I would count pretty well alll the Miles sets (Columbia CDs and Mosaic vinyl) in this category plus a whole raft of Mosaics.