Depends on your family history. Does your family have a history of any complications - premature, multiples, low birth weight, etc.? If so, then just head straight to your local Maternal and Child Health Clinic, register with your HKID, proof of address and proof of pregnancy (I just brought along my positive pregnancy test) and let them assign you to the local hospital. The private hospital will send you to the public hospital anyway if you have major complications, because the public hospitals are the only ones really equipped to handle them. I wish I knew that first time around - would have saved myself a bundle of aggravation and money. You won't get a single OB, as you'll be seen by whoever is on duty, but the upside is that you'll get the right treatment. For example, given my previous history and family history, I'm being seen by the High Risk Ward at Queen Mary Hospital. And by the way, all the public ante-natal visits, tests and scans are free for HK residents. After the birth of your child, you can register him/her for HK$1 at the same MCHC, where they will administer all their necessary vaccinations and chart their development. They also provide up-to-date parenting and breastfeeding advice, and many will actually have a lactation nurse and nursing room on-site. I found them to be much more up-to-date than many private OBs and pediatricians!
I had gone to two different OB the first time around and unfortunately, I wouldn't recommend them. For one thing, the waits were just as long as at the public clinics. Also, the consultations are very short. I got most of my advice from books, other moms, forums, MCHC etc. - not the private doctor. They just don't have the time!