The Mellotron Group

That really depends on if you're talking about the "first wave" of
progressive - when they just thought that they were making cool music and
used the instruments they liked. Subsequent use of things like the Roland
guitar synth, Chapman Stick and extensive use of certain guitars and basses,
like Alembic's also scream "progressive". I think we ought to think about
other types of the gear that made the sounds we all enjoy --- Hammonds are
less interesting without a Leslie and really come to life with a hot or even
overdriven amp - heck, Pinder used a Marshall stack on stage with his MkII.
Also, many groups use ARP's, Oberheims, Crumars and other electronics to get
a distinguished sound as the 'prog' category became overrun.

 

The there's the music itself...

 

Vance...Use of the Chamberlin also created a distinctive sound from the
'tron users..

-----Original Message-----
 For me, the 5 most important instruments for Prog, in this order, are
Mellotron, Bass Pedals, 
Hammond organ, 12-string, and mini-Moog.