Rarest used version must be Version43/Kickstart 3.2 on the Walker =)
It is/was included in AmigaForever though, but on a real hardware it should be rather rare.
OS4 Kickstart 53.23 on the made it to the Limebook Z9 but that's not actually usable so doesn't quite count.
Hyperion still have am IBM E-Lap with a usable (some might question that) version of OS4.0.
https://www.youtube.com/watch?v=Bghlbjmzzqo
