If you're generally speaking in terms of RTW only, I think the general consensus is that Borrelli and Kiton shirts are the best.
If you take cost into consideration, I think that Carlo Franco shirts are seen as an excellent value. Also, it's pretty easy to find Borrelli shirts brand new on ebay or on the Virtual Clothes Horse site for about $110-160. Quite a savings from the $400-525 retail price.
Charvet shirts are also held in high regard, as is Barba, Finamore, etc. Though I have no personal experience with those brands.
Of course, you have to remember that fit should trump quality (assuming you can't have both), as a top of the line shirt (Kiton) that fits poorly will never look as good as a lower priced/quality (Hickey Freeman - not saying they are bad, just lower on the scale than Kiton) if it doesn't fit you well.
Also, know what details you like, and what you don't like.
I'm surprised that nobody has mentioned Hilditch & Key yet for quality shirts. At full price, they aren't that bad, and can be had on sale quite regularly.