All of Op 39 is insanely good!
I played #6 A minor last year for a competition, and I would disagree with whoever says it's the hardest of the set. It was the most difficult thing I've learned given the time constraint, but I'd say that #1, #3 and #9 have some stuff that is on par, or more difficult, than #6.
#3 is magic though, as well as #7. All of them are good!