I decided to call the thread some of my favorite transcriptions because maybe that's easier than "greatest" or "5 favorite" transcriptions. So feel free free to list as many or few as you like.

A few of mine:
Ravel/Ravel: La Valse
Stravinsky/Stravinsky:Petrouchka
Schubert/Liszt: songs(virtually any that Liszt transcribed)
Bach/Feinberg:Largo
Beethoven/Liszt:Symphony #9


And the newest one:
Stravinsky/Sun:Firebird Suite