Luffy- Not good parenting material,.
Zoro- So long as you enjoy being raised to be a swordsman (or a heavy drinker), he'd be perfect.
Sanji- If you're a boy, life with Sanji is likely to be pure hell. If you don't clean your plate, you get kicked into a corner of the galley. If you cook something that tastes shitty, you get kicked into a corner of the galley. If you say something rude to a lady, you get kicked into a corner of the galley. If you get cigarette ashes in the food…you get the idea. If you're a girl, then Sanji is the greatest dad in the whole world until you're sixteen. Then, he's the most embarrassing dad in the whole world.
Usopp- While he's the undeniable "Bed-Time Story Champion of the World", I wonder sometimes if Usopp will be brave enough to tackle the adventure not even Yassop would: raising kids.
Franky- I agree with everything that has already been said about Franky in this thread.
Brooke- I'm not really sure how this would work. Starting his own family is pretty much out of the question, necrophiliacs notwithstanding. The kids at the orphanage are going to freak out when he comes around, and his asking the female staff to show him their panties is liable to get him thrown out the door. About the only way I can think of Brook even having the opportunity to be a dad is for some down-on-their-luck kid to adopt him. He seems like the compassionate type, for a pirate, so as long as the tyke's sense of right and wrong aren't totally at odds with Brook's, he might turn out to be a pretty cool old man to have as your "old man".