I will vouch for Drug War, easily the best action scene from To's whole career (yeah, I went there)
Great, great movie too, I mean, I frigging love Johnnie To and even I didn't expect to love it as much as I did.
and on balance, the best shootout ever does belong to Heat, or Hard Boiled, the finale of A Bittersweet Life says 'I ain't too shabby'.