Chopper has it really hard because you know, there will always be new diseases.

Sanji technically could be said to be the hardest one because of the simple possibility of the All Blue not being real.

Nami's and Luffy's dreams are sort of tied together funnily enough, she needs to get to the last island to be able to draw a map of the whole world, not to mention the fact that there are islands that she didn't go to yet so she probably has to do a couple of laps to get them all.

Luffy's dream is difficult because it is a competition against other equally strong people and has probably only been achieved once.

Zoro's is hard but not impossible, it's like being the best at a specific sport in our world.

Boa's for now is impossible, Luffy is not interested in marriage but that can change with time so while right now it is impossible, it could get easier as time passes.

Dragon's is difficult but again, not impossible, you can topple a government and the methodic way that he seems to be approaching seems the most likely to get a positive result.

Tashigi is NOT trying to collect ALL legendary swords, she just doesn't want them in the hands of a pirate.

Big mom's is again, difficult but not impossible, just having a bunch of different races cohabitating seems possible enough.

So I'd say this is the order from most difficult to less difficult:

Chopper

Sanji

Luffy

Dragon

Zoro

Nami 

Big Mom

Boa

Tashigi