well i think...
Beethoven - String Quartet No. 14 and The Grosse Fugue, and you cannot let down the infamous 9th Symphony
Stravinsky - The Rite Of Spring it was his mos inovative work, but i think The Firebird deserves to be his greatest work.
Brahms - 2nd Piano Concerto and his 4th Symphony
Wagner - the monumental The Ring of the Nibelungs, but for a single opera Tristan Und Isolde
Mahler - i think his 9th symphony is his greatest, but some people argue that the 2nd one was his best
Richard Strauss - An Alpine Symphony i think this symphony makes mozart look like if he was a baby
J.S. Bach - well, what can i say about him, i think he deserves the greatest composer ever award, his best work, humm, could be the Mass in Bm?, or St Matthew passion? the violin Partita No. 2 (specially the Ciaccone), the Brandenburg Concertos?, i will go for St. Matthew Passion
Sibelius - Violin Concerto in D minor one of the greatest of all time
Tchaikovsky - Violin Concerto in D Major or his Symphony No. 6, even though i dislike enormously Tchaikovksy
Bruckner - Symphony No. 7
i think these are the most complex and complicated music pieces ever composed