Hong Kong for sure. Best tailored suit, cheap and they will make it anyway you want. Bring pictures of suits that you like from famous designer and they will copy it!

Hong Kong is wel known for its excellent tailoring. But excellent tailoring doesn't come cheap. I've used Maxwell's but there are many others as well.

Cicerone, who lives in Hong Kong has posted a list of recommended tailors. Use the advances search function to find her posts.

If Eschew thinks tailoring in Hong Kong is cheap, I can only assume that it has been many years since he was last there.

Cheap in comparison to what you can get here in North America ... and if you can get someone local (who can speak the language) to go with you, it would be even cheaper as they do charge the tourists more.

I will have to dig up the name of the shop and address in Kowloon now.

http://www.fodors.com/community/asia/hong-kong-custom-suitsin-hk-and-at-home-nytimes-articles.cfm

A quality suit made with Italian Scabel or Zegna 150 Superfine fabric will cost %800+ based upon a recent vist of a friend to Maxwells. A cheaper fabric but still quality 150 will be $600 ane requires 3 fittings and takes 3 days or squeezed maybe 2.5. I know nothing about Beijing.

I had a few suits made in Beijing. I am very happy with the quality. Only slightly inferior than the ones made by my Italian tailor back in Canada but for 10 times less.
I go to Ya Xiu market, third floor facing the escalator. They have a wide variety of quality fabrics. They have tons of magazines for you to choose your model or bring one suit you like. If you bring a suit you like they can improve the turnaround and you will need less fitting sessions. It could be fine on the first try.
Turnaround is quick, somewhere between 1 and 3 days (one day if they copy your existing suit). You need to bargain hard to get the best price. I am still wearing the ones made 3 years ago so I am not totally sure with current prices. I had a suit made with 3 pairs of pants (so 4 pieces in total) and the cost was 1400RMB one time and 1500RMB the other time. At that time it meant $200 but the way exchange rate has gone it now means $250. With inflation in China I would expect the price to be higher but still less than 2000RMB.