Essential Reich....
I would have to cast the vote for...
'Music for 18 Musicians' : incredible subtle ambient variations of a
pulsing theme. New sounds come in so subtly that you don't notice until
it's almost done
'Electronic Counterpoint' : you'll know it as 'that part from Little
Fluffy Clouds'
'Come Out and Show Me' (I think that's the name) : excellent early tape
loop experiment.
As pointed out, if you can afford get the 10 CD set on Nonesuch. All his
works in a nice package. It's a permanent fixture of my bedside CD player.
Jeff