The Strawhat Pirates (Mugiwara Kaizoku-dan) may sail under the flag of their captain Luffy, but each of them also have their own personalized version of the Jolly Roger!
From the Trivia section of each of the characters’ entries in the One Piece Encyclopedia wiki.
Captions follow images.
Luffy’s flag, featuring his straw hat. Also used as the flag for the Straw Hat pirates as a whole.
Zoro’s flag, featuring his three swords and bandana.
Nami’s flag, featuring her early blouse pattern, her cheeky wink and tongue, an OK hand sign (meaning money), her hair curls, and a flash that perhaps represents the shine of treasure.
Usopp’s flag, featuring his nose, bandana, and slingshot and star (his slingshot rounds usually feature the word ‘star’ in them).
Sanji’s flag, featuring his swirly eyebrow, a chef’s hat and dining utensils.
Chopper’s flag used to be Dr. Hiriluk’s, featuring the pink sakura that gave Hiriluk the hope to keep on living.
Chopper’s new flag basically looks exactly like him, with sakura nearby.
Robin’s flag, featuring her cowboy-style hat and Hana hana no mi conjured arms.
Franky’s flag, featuring his gangster look, hairdo, triple chin and mechanical parts. (Alternate one drawn by Oda at here.)
Brook’s flag, featuring his goofy look, afro, top hat and cane. It probably helps that he himself is a skeleton.