I'd say that the Genesis/MD by far has the most different licensed variations because you have
Genesis 1 high definition graphics model
Genesis 1 with the lockout chip (mostly to prevent you from playing unlicensed games i.e. Accolade/Ballistic's early releases)
Genesis 2
Genesis 3
The new "sonic head" one
MD 1,2
CDX
X'eye
Wondermega
Wondermega 2
Nomad
Tera Drive
Mega Jet
Amstrad Mega PC
Aiwa Mega CD
Samsung Super Aladdin boy
LaserActive PAC
not to mention the various x number of games in one plug and play units though I think counting them as console revisions is really splitting hairs.

I don't think that the Atari 5200, 7800 or Jag ever had a hardware revision but someone more steeped in Atari knowledge would probably correct me.