The soundtrack "Northstar" (sometimes called "Etoile Polaire") was Glass' crossover breakthrough for rock fans. If you're coming at his music from that angle, this is a good start.
I saw his ensemble perform the piece live sometime in the '70s and took away one message:
Play this loud!
If you're not coming at this from a rock fan's perspective, Stvecham's choices are spot on-
Glassworks and the Photographer being the prettiest music (to my ear) and
Einstein being the most ambitious.
Enjoy!
Marty
PS For a further rock music connection, Glass collaborated with the band Polyrock on their first LP. This is a great rock record that strongly echoes some of the themes heard on North Star.