Górecki's Concerto for Harpsichord and String Orchestra (op. 40) is astonishing music. The layering of harpsichord and a deep string orchestra makes for some very complex texture that he accents with intricate rhythms. I love the simple melodic structure of process music like this concerto but I can't begin to sort out the harmonies here.
If you've heard of Henryk Górecki it's probably because of the Dawn Upshaw recording of his Symphony No. 3 that achieved huge popular success a few years ago. Where that symphony is haunting and lyrical, this 1980 concerto is forceful and propulsive.
Amazon has a sample of the recording I have, see track 5. The dizzying introduction gives a good flavour of the complexity of the piece. For minimalist music with relatively limited instrumentation there's a lot going on.