Denim, but make it couture! I just hate the idea of all the new denim jackets produced every season - it's so wasteful, especially when denim is such a durable material that will keep forever.
Vintage denim is especially durable because it's 100% cotton. More modern denim often has an elastic fibre in it which deteriorates and shortens the life of the jacket but also makes it impossible to recycle.
I've had a thing for Levi's jackets since I was a kid - I once had one stolen from me in a nightclub and I still think about it! I get such a kick out of creating completely unique garments using fine fabrics and vintage pieces.