All the networks here (apart from China Mobile) offer 3g plans.
3HK is probably the most competitive in terms of price and services offered. Some people have complained that their service isn't the best, but then again others have not had a problem with their services.
CSL, I have found, to be expensive. About 6-12 months ago people were also complaining about their services, but I think they have improved.
Smartone-Vodafone (which is what I have) seems to have a decent following. However, recently, I've found their service levels to have dropped.
There is also PCCW, but from reports they are probably best avoided, due to their cowboy attitudes to signing with them.
I should add all the networks offer plans that allow for unlimited data and also tiered plans (i.e. max of 100, 250, 500 mb, etc data). Just go on each of the company's websites and the information is all there.