definitely op 25 no 12. It is beautiful and not too difficult! I am working on op 25 no 5 now. I really love it! So far I know op 25. no 2; no 11 and op 10 no 12. I cannot play through op 10 no 12 as well as the others though. I learned some fingering wrong and so I avoid playing it. Some of the op 10 are really quite difficult, I think because the tonality is so different. To me, the op 25 is more musical batch of the etudes.