I have heard the recording of all Faure Nocturnes, but studied only a few of them, so I won't ranking them all.
I do know that most people consider Nocturnes Nos. 3 and 4 the most accessible, and the "easiest" to enter the very elusive and beautiful world of Faure's music. Nos. 2 and 5 are wonderful pieces, but their middle sections present much more advanced technical challenges than Nos. 3 and 4.
No. 6 is deservedly popular, and it is generally considered a milestone where Faure matured and codified his own unique style.
It's difficult to judge Nocturnes Nos. 7 to 13 because they are so "Faure"--i.e. the style too unique to generalize one way or the other. These are highly subjective and you won't find any consistent opinions on them. And in most cases, I believe the difficulties would be musical rather than technical.