What is the power order of the Straw Hats? I know it's quite difficult but I'd like to know the power level order between the Straw Hats. Currently I am assuming that the order is something like this: 


Luffy 
Zoro  
Sanji


I'm aware from place 4 onward it gets really difficult, but maybe some hints exist. For example, Oda said that Usopp's current power equal that of Zoro before time skip. That would be a good example of a great hint.

Some measurements could be the opponents they have faced, their individual power, devil fruits and for those that have it, their haki.
This is after the dressrosa arc:

The bounties are a good representative of the strength of each member of the strawhat crew.

The only odd ones are Sanji, Usopp and Chopper.

Chopper because he is viewed as a pet instead of a full member
Sanji because he didn't participate that much and Usopp because he was said to be the one who instigated everything but that was wrong.

So for power range according to posters (with dismissing odd things):


Luffy
Zoro
Sanji
Nico Robin
Franky
Brooke
Usopp
Chopper
Nami


Usopp, Chopper and Nami are also called the weak trio so saying those three are the weakest is a pretty good assumption.