Comment by argimenes

Probably the last truly great classical improviser on piano was Anton Rubinstein (1829-1894). His Fantaisie in E minor, Op. 77 is probably the pinnacle of his skill in (notated) improvisation.

After that, the great classical improvisers were probably organists in the 20th century, like Marcel Dupré (1886-1971) and Pierre Cochereau (1924-1984). Cochereau created vast, highly personal, symphonic forms with dazzling orchestral registration.