China Petroleum (specifically, I use the one by the entrance to Tung Chung Road) gives you $2.1 off per litre at the moment with their loyalty card. If you don't have one, the staff are happy to let you use theirs.
At Sinopec I often find similar discounts, though it does vary between branches I find. I've found cases where their regular petrol was discounted $0.9 only, but the premium one by $2.x, which made it cheaper than the regular. Bizarre.
Am keen to see what discounts the other brands offer, as I hear other drivers claim they get better performance - e.g. with Caltex or Shell. Anyone ever found performance differences?