A/UX is an interesting one, for one, it's Apple's original Unix release, which predates Darwin/Next by several years. The other neat thing about A/UX was that

A/UX is an interesting one, for one, it's Apple's original Unix release, which predates Darwin/Next by several years. The other neat thing about A/UX was that it had a utility that let you format regular SCSI drives so that consumer-grad Macs could use them. This greatly extends the life of of System 6/7 devices as Apple-branded SCSI hard drives bigger than a couple megabytes from the early 1990s are super rare now. System 6 is pretty snappy even by modern standards (was written in assembly) and has backports for IE 3/4, IRC clients etc.