1 Usopp, for sure a good marksman and I guess he can be a good teacher as well.
2 the master young Zoro had at the dojo (I forgot the name XD), I don't know if he actually is a strong swordsman but for sure has a long and solid experience in teaching since that is his job.
3 here I think I would also look for some fancy martial art. Not a fishman since fishman karate is for fishman only, or at least that for sure goes best with fishman while results with humans would be subpar, maybe Sabo with his dragon style could be good, Sabo also could be a fine teacher but the problem is that he is still young and probably with no experience in the field... Dragon??? From whom did he learn his style?
4 Eh, this is a problem since every df is unique and even if you are WB who mastered a strong df and was regarded as WSM how are you supposed to know even the basics about.. let's say Buggy fruit? Luffy too learnt his gears on his own (but that is plot power). The one that I see with the greatest knowledge about DFs is BB but I wouldn't want him as a teacher (he could discard you anytime if there is a gain and I don't see why he should teach an average human, and even if you have extraordinary potential that is even risker cause if he teaches you there must be a reason... I guess that here is a process of try and fail on my own.
5 Ray seems the perfect choice, for sure very knowledgeable about the subject, seems quite a good teacher but first you will have to unlock haki and that is already a long journey and, like for Sabo/Dragon this is a VIP teacher and I doubt he will have the time and will to spare (note of realism).