These would be my personal top choices for these categories (and I have bought from several):
Colored diamonds: Diamonds by Lauren
Superideal cut round and princess cut diamonds: Whiteflash and High Performance Diamonds
New antique style cut diamonds: August Vintage, Inc (owned by Jonathan who used to be with GOG), Victor Canera, and GemConcepts has a limited amount of specialty cuts available
Pave handforged settings: Victor Canera
High quality antique (and other) style settings and excellent design: Caysie van Bebber (CVB Inspired Design)
Good quality settings at a lower price point (but you need to have a clear idea of your design and be able to verbalize and visualize changes needed in the CAD images): David Klass
Now if what you are searching for doesn't fall into any of these categories, I can add others, perhaps!