A bit late to the party, but as mentioned, Koyaanisqatsi is his most seminal recording; it should be paired with the movie for full effect. However, whenever I go into Walmart, I usually have this album in my earbuds as a survival strategy.
Glassworks and Northstar can also serve as a good introduction. Einstein on the Beach - based just on its thematic complexity and scope - should best be saved once you’re familiar with Glass’ style.
I have several of his symphonies, but they never seem to have reached the visceral impact as those other three recordings. YMMV.