W. W. Chan & Sons are the best in town. Pricey, though. HK isn't a cheap place for tailors any more, if you want any decent work. Walk down Nathan Road and you'll be harangued by endless Indian chaps offering you a HK$3500 (£300/US$500) suit, but the fabric will be sh*te, and the tailoring sh*te. If you have the budget for a £1000 suit, go to Chan.
For shirts, I'm off to check out Jantzen after the Sevens debauchery is over. I'll let you know what they're like. I've heard bad things about them for suits, but for shirts, they're apparently the bee's knees. i have a great 20s shirt to get copied, so I hope they can do a good job. At about HK$400, they're great value. Let's see if they can take instruction.