Straw Hat: Loyal to an extent, but they are untested (with the exception of Usopp, who are currently under test). Zoro had said that if Luffy gets in the way of his dream he won't hesitate to kill him, and with Sanji, we still don't know what he'd do when he finds All Blue.
Usopp: Loyal, but for a brief moment they lost faith in him.
Buggy: When Buggy was MIA, Mohji and Cabaji immediately fought for Captain position. Perhaps not that loyal, but more afraid of their captain's power.
Kuroneko: Well, they turned against him at one point, although they're quite loyal for waiting 3 years.
Don Krieg: Gin is very loyal despite treated badly. The others are fairly loyal I guess.
Shanks: Loyal. His crew respects him, and most have been with him for a long time.
Okama Way: Loyal, and seem to genuinely care for Bon Clay. They're with him even after Baroque Works are defeated.
Foxy: They love their captain! XD But somehow I doubt that all of the members stolen from other crews are 100% loyal…
My pick: the Okama crew.