It's been my experience that the urethane orings do last the longest. I'm actually running a mix of urethane and HBNR right now. I have urethane for the orings responsible for cycling (rear can, bolt sail and inside of back latch) and HBNR for the other dynamic orings (front and middle can and bolt guide).
Formerly of DBS Kidz 2, DBSPaintball.com and DBS Titans
I've used HNBR before, and they work. But contact an oring manufacturer, and they'll tell you HNBR is wrong for the application. HNBR are still better that regular Buna, but for dynamic seals, Viton and Urethane are better. Urethane and Viton both have less friction once broken in