Favorites, off the top of my head:
Custom grip-makers from years ago, top-of-the-line: Walt Roper (Matheis Gagne), Lew Sanderson, Charles Wendell (Kearsarge), Roy Fishpaw
Custom grip-maker from years ago and that anyone can afford: Steve Herrett
Contemporary custom grip-maker, top-of-the-line: Keith Brown
Contemporary custom grip-maker anyone can afford: John Culina (Kurac here at the forums)
Up-and-coming and worth watching: Schofieldkid81 (forum member)
Notable mentions: Don Collins, Patrick Grashorn, Dave Chicoine, Craig Spegel, all of whom are at the top of their game in their respective niches
I don't care much for mass-produced cookie-cutter aftermarket grips when there are still so many true craftsmen like the aforementioned gentlemen putting real hand-crafted wood (or stag, ivory, etc.) to steel out there. You can't go wrong with any of their work.