I always use
AXA , been very happy with them. Premiums are pretty good and we have had to make a few claims which were settled promptly and with no fuss. Cost for 90 days is between HK$761 and HK$1,168 depending on the level of cover.
AXA can cover you for a maxium of 6 months for a single trip.
Another option is
IPH. They offer very good annual worldwide cover but there is a limit of 90 days per trip which may not suit you. Cost is around US$138 which is very good but as mentioned there is that 90 day limit.
Send me a PM if you want to chat about the above