I've said it before, I'll say it again: shirtmaking isn't rocket science. If you can perfect your pattern with Jantzen over three or five shirts you're well there.
That said, I'd suggest using a good shirtmaker locally or somewhere that you visit often. I used Charvet's bespoke service and was happy with them. I also used Lanvin and Courtot. My comparisons are elsewhere on this forum if anyone is interested, just dafs. However, I cannot emphasize enough that if you are going to put down $500 or $1000 or however much the top-of-the-line bespoke shirts being discussed here might cost, you should take advantage of what the shirtmaker offers: Go there in person and have the shirtmaker himself or herself measure you, not a salesman. Charvet and Lanvin provide a sample fitting shirt which is used to perfect the pattern and which is not part of your order -- most British shirtmakers don't do this, and it generally takes a few weeks . You need to be around for that fitting. When your actual shirt order is finished, visit in person, have the shirtmaker assess the shirt -- s/he may have additional adjustments to make. Charvet also advises you to wear the shirt over the course of a few weeks or a month, wash it a few times, then come back with it in case any further adjustments are needed as the shirt breaks in. Lanvin was happy to do this too. If you are a real big-timer or a pretentious blowhard perhaps you can make it to Paris often enough to make it work. If not...
Many more reasonably priced shirtmakers may allow you to make adjustments too. But there is no point in being measured by a salesperson who doesn't know a thing about how a shirt is put together and paying an enormous price for an item you will not be completely satisfied with. The bespoke or MTM services offered for Charvet out of Bergdorf or Saks are stock specials, no matter what they say, and the prices for Charvet RTW and stock specials are much, much higher in the US and UK than they are in France, although even in France Charvet is very expensive.
What is important in a custom shirt is fit -- that you get it the way you want and in a way that accomodates your body. Construction, of course, is another priority. Then should come things like cloth selection, handwork, and all the rest.
I used Charvet at a time in my life which was very different in almost every way from today. Even then the price was unjustifiably high, but a wonderful indulgence. What was I paying for? Dreams, the incredible cloth room, the experience, service and the reliability of product and fit -- more or less in that order. YMMV.