(BTW, I have this same post in Piano Society and just wanna share it in this thread.)
Before, I taught I could just transpose any piece in any key that I like; but I realized this is easier said than done. I learned from my teacher that each key signature has its OWN set of "PLAYABLE" passages that is BEST (if not ONLY) suit to be executed in that specific key signature. For instance, the left-hand passages in Revolutionary Etude is almost impossible to play when the piece is transposed to C#m. Or just imagine a Fantasie-Impromptu put in Cm. In this case, a greater strech is needed for the left hand and the right hand is another story.
Thus, the conclusion is, there are passages that is PIANISTIC for one key but UNPIANISTIC for the other. (We say "pianistic" to mean how EXECUTABLE a passage is as far as FINGERING is concerned.) This is primarily the reason why often you will notice SIMILAR passages among pieces having SAME key signature (even if they are of different compsoer).
This theory cannot be easily noticed yet in Baroque music but is very obvious in Classical and Romantic music whose compsers are great geniuses that they know what key is best pianistic for a specific piece they compose - especially Beethoven, Chopin, Liszt, and Rachmaninoff. However, some Modern/20th Century composers became unpianistic, somewhat deviating to this norm by composing "FREE FORM" music such as ATONAL pieces - pieces not necessarily having a specific key.
All in all, I learned that it is not really true that you can just transpose any piece in any key you wish. In fact, it is unscholarly, unprofessional, and unpianistic to do so!
Now, I believe the best key signature is the one having the MOST pianistic and playable passages. The obvious contenders are Cm, Eb, Fm, Ab, Bbm, and Db. But since they are overused, I prefer to like underrrated keys such as C#, D#m, and F#m (or Gbm). Anyway, I vote for B and its relative key G#m/Abm as my favorite keys! (BTW, the G#m/Abm scale is the hardest scale to execute for me!)
