Completely depends on what you're looking for. Motomillion is good. I ordered a Fullsix carbon fiber gas tank and Manni actually called me to make sure I wanted the matte since it doesn't quite match Ducati Performance matte, which is more of a satin.
Fullsix for carbon fiber by a long shot.
WRS is good. Best windscreens by a wide margin.
HSBK / Ducati NY is awesome as well. Probably one of the more legit dealers and aftermarket race parts sellers. Seem to be quicker than other dealers for getting OEM parts.
motovationusa.com get's a HUGE nod. I ordered some Ilmberger CF parts, upper sides and a nose and the uppers arrived with some damage. They sent pics about it and were very upfront. I opted for a refund since it would be another 8-12 weeks or so to get replacements.
Alex at mygpracing.com is my first go to place for bespoke items.
He only carries the best and is always the utmost in courtesy and decorum.
Always a rewarding pleasure to work with him.
Do you want aftermarket upgrades or an alternative OEM supplier? Omniaracing are pretty good, but 10 minutes on google searching for specific parts usually gets you a list of reputable suppliers then you can work out the best deal for your situation.