Ditto on William Cheng
Like Vulcan, I found Wm Cheng & Son on Hankow Rd, Kowloon, to make very good quality shirts & suits. I originally found Cheng in the early 90s by way of references from Australian barristers who used him. The shop is on the 8th floor.
I have also used one of the high end tailors in Kowloon, Y. William Yu, 46 Mody Road. My grand frather & father have used Yu over many decades. I found their suits to me very superior & they have a branch in New York City, 521 Madison Ave. This is a tailor who will pay serious attention to all details & insist upon proper number of fittings. They do make the circuit every year in the USA.