hi, I can't really advise on how difficult they are, except I played number 8, which is not his greatest but is technically easier than some of the sonatas. Lest I get flamed off the planet, I hasten to add that it's a cow to play in a musically decent way.
FWIW my ranking of which are the most beautiful would be (in VERY rough order within each line):
-Greatest: 23, 24, 20, 25, 21, 22
-Nearly as great: 27, 9
-Slightly less great: 19, 26
I think 25 is an underrated masterpiece. The F major section in the Rondo is a piece of beauty I can't live without. You'll know which bit I mean when you hear it.
14 is rarely played but its slow movement is almost as beautiful as 21 in my book. You can get the basic idea at amazon or Naxos.com.
I remember liking 17 & 18 but I haven't heard them recently enough to rank them.