to the OP the two best providers are probably CSL and Vodafone Smartone.
CSL has two brands - one2free and 1010. 1010 seems to be more geared towards the corporates and one2free is more towards individuals.
both CSL and smartone are a bit more expensive then the other providers, but you can still find some good deals with them.
you then have peoples, 3 and pccw.
3 and pccw seem to have lots of customer complaints. peoples (china mobile) seems to be generally fine but does not have a 3g network.
to gomanred - 3 is the provider with the iphone deal in hk. with 3 you get an 18 or 24 month package with unlimited iphone internet access, etc. however you can buy iphones unlocked here and put any network provider's sim into them. however to fully utilise the iphone, you will need to find a deal which will give you a generous internet allowance.