IMHO, best editions of some composers:
Bach -- Neue Bach Ausgabe, Breitkopf und Hartel
Handel - Henle
Scarlatti, D. - Schirmer (Kirkpatrick)
Haydn - Vienna Urtext (Landon)
Mozart - Presser (Nathan Broder), and Henle
Beethoven - Henle
Schubert - Henle
Schumann - Henle
Chopin - Paderewski, and Vienna Urtext.
Liszt - Editio Musica Budapest
Bartok - Boosey & Hawkes
Rachmaninov - Boosey & Hawkes
Debussy - doesn't really matter, although some newer editions by Alfred and Peters are good
Ravel - Alfred (Bricard)