Support: Mediator has most SW support.
Stable: I do not know...
Fastest: In some cases I think GRex is faster than some Meadiator, but it depends on a lot of things.
According to DCE the GRex is not in production any more ( unnecessary flame torch removed ). (or at least that was the situation last autumn)
It seems that AmigaOS4.0 supports Mediator and Prometheus while MorphOS1.0 supports GRex and Prometheus.
Radeon support via Picasso96 is being built for Mediator and Prometheus. CGX Radeon support is being built for GRex and Prometheus.
(( unnecessary flame torch removed ))
For the rest, check vgr.com and elbox.com for information about the support of those boards.
