Need a Johnny Cash @ Sun Collection?
The digital only release entitled Johnny Cash at Sun is a superb collection, released by Sony in 2014, that includes 88 tracks. It's my top recommendation for anyone searching for a one-stop-shop when it comes to collecting Johnny's Sun material. It doesn't have every version of every track, but includes at least one recording of each Sun track along with numerous undubbed, alternate, and outtake recordings from these sessions. Fantastic!
Need a physical product? Stick with Time Life's The Complete Sun Sessions 1955-1958. The mastering is as good as the Sony set only with a few less tracks (61 total). If you need just one good sounding version of each Sun track -- this set is for you! Need an expensive runner up? The Varese Sarabande box set from 1999 sounds good and features plenty of bonus tracks.