I've played a bit through a few of Faure's nocturnes, though I have never formally studied them. (So take this with a grain of salt.) The ratings are both rough and subjective.
Hardest: 2,5,12
Moderate: 1,6,7(this piece is much longer than the others),13
Easier: 3,4(has a few harder polyrhythms though),8
Easiest: 9, 10, 11
His 'easy' nocturnes are not exactly easy...they're just not as hard as his other ones. A lot of his earlier nocturnes start out relatively simple, but develop much more in the middle section, and then go back to the simpler melody à la Chopin's nocturnes. (ternary form)
The nocturnes from the 6th-7th onward are a little different. Faure wrote the 6th one after a six year break from composing for piano, so the style will obviously be a bit different then his previously more imitative compositions.
The beginning nocturnes are technically harder and by no means musically easy, but the later nocturnes, while somewhat technically easier, are, I suspect, harder to interpret musically.
My favorites are 1 and 5. I would suggest playing 8, though. It is one of Faure's later nocturnes, meaning it is more characteristic of his style; it is not too difficult, considerably short, and has a
comfortable warm lyricism about it.
If his nocturnes seem too difficult, maybe check out his barcarolles--I think they're easier.
Best of luck!