There are too many to list. It really depends on your price range, too. Right off the top of my head I would say Kevin Cashen and J.D. Smith.
Kevin Cashen is an obvious choice considering he is the first and only person to win an ABS cutting competition with a pattern-welded blade--low layer cound at that. He uses O1 and L6 primarily (two great cutters that cut great together!), and pays strict attention to time and temperature. He also regularly produces pattern-welded hunters that see real use.
J.D. Smith is my teacher, which, on the one hand, makes me somewhat biased. But, on the other hand, it also means that I have seen him construct countless billets of every imagineable sort of pattern-welded steel, and seen these billets go into service. J.D. has an interesting dual classification system; he distinguishes between "weapons grade" and "furniture grade" damascus. Any shortcoming in terms of construction (a very slight flaw) or materials (low carbon steel, nickel, etc.) is cause for a "demotion" to "furniture grade." This steel will only be used for guards, butt caps, etc. "Weapons grade" damascus is the only thing that makes it into J.D.'s blades--be they long swords or short hunters--and is free of flaws and composed entirely of high carbon steel (in this case primarily 1084 and 15n20).
John