Being one of the large 'ALL' islands, this one Still does not seem to be the most difficult, because modern composer differences sound more pronounced than for example Romantic. [All Modern]

Compare two modern tonal composers: german Hindemith and french Poulenc.

Poulenc sounds romantic, simple, diatonic.

Hindemith sounds more energetic, rhythmic, experimental, neoclassical, repetitive, wind instruments. Although he is tonal, he is not diatonic.

Only Hindemith has piano song, trumpet sonata, clarinet sonata, viola sonata, solo piano in this game.

Only Poulenc has vocal with orchestra, oboe sonata, harpsichord, organ in this game.

It seems to be most difficult to guess orchestra pieces. (Hindemith, Poulenc)

Ives has both simple american popular hymn-like melodies and microtonal, polytonality, polyrhythm, tone clusters.

Only Ives has symphony+vocal, piano song in this game.

Only Poulenc has harpsichord, strings+organ+timpani, vocal church music.

Warning: Ives symphony has vocal. (Ives, Poulenc)

Messiaen sounds rhythmic, repetitive, atmospheric, oriental, tonal percussion, bird melodies.

Poulenc sounds simple and diatonic.

Only Messiaen has solo piano, piano song, solo organ in this game.

Only Poulenc has organ+orchestra, harpsichord+orchestra, piano+oboe in this game. (Messiaen, Poulenc)

A simple island.

Poulenc dounds diatonic. Webern sounds atonal.

Beware of the early Webern pieces, which sound diatonic and romantic: Two Early Pieces for Cello and Piano, Im Sommerwind, Langsamer Satz, Passacaglia, Rondo. Also, some of his pieces can sound significantly more diatonic in some places (e.g. Satz op. 5). (Poulenc, Webern)