I second Scott Johnson's recommendation of Jenny at J&H Bespoke. I have only tried couple of tailors in Beijing as I get most of my shirts and suits from Ascot Chang (Hong Kong). Jenny speaks good english and is willing to add many details to suit customers' taste (what a refreshing experience compared to some tailors in silk market). She also offers good recommendation if you don't have a clue what you want. I have 6 shirts made by her and they have been good quality. I recommend opting for imported fabric (rmb400+) as they are easier on the eyes, nicer to touch and easier to iron. The shirts are not Ascot Chang quality due to the fabric used but they cost only a fraction of the price of an Ascot Chang shirt. I haven't yet had any suit or blazer made at J&H so I can't comment on her suits.
If you bring in a shirt that you like or a photo, Jenny can also source the fabric for you based on your budget.
I understand there are many tailors in the same area and competition can be fierce. Hence the sometimes contradictory reviews and comments on the same tailor. You can check a tailor's skill by having a shirt made with the cheapest local fabric (150-200RMB) before commiting yourself fully.