Is there a ranked order for the Phantom 4?
On Aria's Fairy Tail Wikipedia page it states that "he is considered the strongest member of the group". 

This by no means concrete evidence that he is indeed the strongest, but he did manage to sneak upon Makarov and disable him for a short time. So as for official order there really is none, they were all S-Class mages so in terms of strength they were at least on the same playing field. So an example "ranking" could be: 


Aria 
Juvia 
Sol 
Totomaru


This is by no means the correct way to rank them and it really depends on your personal opinion.