Good Parents
Robin: Mature, patient, already the crew's "mom" as it is.
Jinbei: Wise, authoritative, but knows how to care for people.
Sanji: Kind, good motivator, likely not to repeat his family's mistakes (probably a lot nicer to his daughters though).
Probably Good Parents
Chopper: Compassionate, would have child's best interests at heart, but a little naive.
Nami: A little rough when she is mad, but she treats kids with kindness.
Franky: The cool dad; eccentric but tons of fun.
Zoro: The manly dad, would instill confidence, but doesn't display love outright.
Usopp: Tells tall tales; but otherwise would probably be decent (or at least have the sense not to bail out on his kid like Yasopp did).
Just, no...
Luffy: Fun, but walking disaster.
Brook: Crazy; doesn't seem to have the attention span to raise a kid.