The nocturne you mentioned is probably the easiest (BUT NOT EASY).
Of the Scherzi you mentioned, I'm told that 1 is the easiest, but I'm guessing it's because of the lack of actual music you need to learn (About 2/3 of it is repetition) and the coda is not that big a monster compared to say, the 2nd ballade.
Scherzo2, I bang through that piece just for fun. The only real hard parts are in the middle I find, but that's just while I bang through it so don't take my opinion too heavily
Scherzo 3, I actually found this the most difficult to read through besides 4, Slow sections need a bit of work with the descending cascades of notes. And of the first 3 scherzi, I found 3's coda the most difficult.
Scherzo 4, I'm told it's the hardest one, though I've never taken the time to read through it.
Ballade 1, I've read through, and obviously the biggest technical challenge is the coda, though the 2nd page is fairly difficult technically. Musically this piece is a monster (all of the ballades are). I've seen way too many butchered 1st ballades, though that might just be due to the popularity.
Ballade 2, More difficult than 1 and the coda is an absolute biotch (DOUBLE NOTES EVERYWHERE).
Ballade 3, the only ballade I've actually learned, of the 4 ballades it's the easiest (I'd say same difficulty as the 3rd scherzo). Most of the technical issues come at the end, starting with the C#m section. Another musical monster, but I don't think it's as bad as the other 3 ballades
Ballade 4, Declared by most to be Chopin's most difficult work (Although my teacher thinks that the Barcarolle is more difficult). Technically, it's HARD. Harder than any of the etudes. Except it's main difficulty lies in the fact that it's one of Chopin's most mature pieces and so musically it's one of Chopin's most difficult.
Barcarolle, not as technically difficult as the 4th ballade, but playing well requires absolute mastery over the double notes (not fast, but they are required to be absolutely smooth and melodic). Another one of Chopin's most mature pieces.
Polonaise-Fantasie, I didn't find technically difficult. I don't know much about the piece, but it's late Chopin so assume the worst.
Polonaise 53, technically difficult, musically not as intense or deep as other late Chopin, but it's a beautiful piece and must be played thus. Not much to say, there's lots of octaves in the middle section, and a bunch of jumps and scales and whatever.
Polonaise 44, I read through it, I'd say it's easier technically than the Heroic polonaise, but more difficult musically
Fantasie, Technically, it's got a few issues, but lighter than the Scherzos and the ballades. I'm told it's easier than all of the scherzi and the ballades