There are a ton of great builders out there so it's more a matter of what floats your boat based on material, construction, location, etc.
What are your criteria? Do you want to visit the builder in person? Do you have a good fitter that can work with the builder? What do you want aesthecially? Traditional lugged steel? TIG-welded steel with carbon? Ti bead-blasted, painted or brushed? Carbon? So many variables that you need to narrow it all down for recommendations.
I just received a custom Ti frame from Tom Kellogg at Spectrum. It's fabulous. Everything I wanted. One of the few ti builders who will brush the tubing rather than bead-blast it. He also does TIG and lugged steel. Tom has been around for decades and is considered one of the best. Up there with Sachs and Walker.
At the other end of the spectrum (no pun intended), I'd love a Kirk Terraplane for a lugged steel frame, but he'll also do TIG. Joe Bell does his paint.
When you see your bike in your head, what do you see?