On an ongoing unlimited basis the two best are the Citibank Premier Miles and the Amex Cathay co-brand.
The Citibank has the advantage of being able to credit miles to different schemes (AsiaMiles, Thai, NW WorldPerks, SQ KrisFlyer), and that the miles don't expire as long as you just hold them on the card. There's a fee waiver for the next couple of years, and past experience says Citi will continue to waive if you threaten to leave. This card also gets you access to the Premier Plaza lounges at HK airport.
The Amex has the advantages that it has no fee provided that you are CX MPC Silver or above (or spend HK$150,000 per year on it), that it comes with good insurance schemes (travel and purchase protection) for things bought with it, and it has some bonuses for purchasing CX flights online. The downside is that it's only AsiaMiles and they are transferred immediately, meanin that they enter the AsiaMiles 3 year expiry cycle. The Amex also comes with a free Priority Pass card for the Primary card holder which is nominally worth a couple of hundred US$ per year if you fly a lot in ways that don't otherwise give you access to airport lounges.
Both cards have base earning rates of HK$8 per AM for local spending and HK$4 per AM for overseas, and both currently have promos going on which give you up to double that rate, plus sign-up bonuses.
There are some other cards which give HK$6/AM up to certain limits - DBS is one I think - but I prefer not to have to worry about managing the limits and juggling too many cards, so I just have the two I have mentioned above (plus my HSBC Premier Mastercard, which I very rarely use since it is only HK$12/AM).