There are many excellent bespoke shirtmakers. A list of London shirtmakers could include:
Frank Foster (my shirtmaker)
Turnbull & Asser
Budd
Emma Willis
New & Lingwood
Hilditch & Key
Harvie & Hudson
Sean O'Flynn
Oscar Udeshi
And they're all roughly the same quality and all have very talented people making the patterns and the shirts. The main difference is in style, and even there the differences in most of the shirtmakers are small. Some have exclusive fabrics. I like Frank Foster because they have a unique style that appeals to me, and they use fantastic interfacings in the collars and cuffs. Best shirtmakers comes down to preference.
Outside of London are many other incredible shirtmakers, famously Charvet.