The Mosaic has far and away the best sound on the pre-war Basie Columbias with Young, but he left in 1941 to be replaced by Don Byas. The easiest way to get all those is with the Definitive sets, but, well, objections to that particular label have been amply discussed here. If you want to avoid that label, you can go with the Classics issues, but they can be spendy to acquire. I've been on something of a crusade to round up all the available pre-war airchecks lately. They're spread out all over the place. Recently got the Chatterbox things, but the sound is pretty bad on those. The Savory material is absolutely essential, IMHO.
gregmo