These are in chronological order (not order of importance - I couldn't argue that one!) and relate quite closely to my previous list of 7 greatest composers. Again, I have listed pieces which, in my humble opinion, were seminal in the development of classical music.
1 Guillaume de Machaut - Messe de Nostre Dame
Dating from the 1360s, this was the first independent setting of the Mass and the first known piece of its scale ever written. Machaut can safely be credited with being the first large-scale 'classical' composer and his example served as an inspiration for generations of composers to come.
2 Monteverdi - Orfeo
Scholars argue whether it was Peri's 'Daphne' (1597) or Monteverdi's 'Orfeo' (1600) which was the first true opera. Peri's piece is lost, sadly, so we cannot tell exactly what sort of work it was. Monteverdi's masterpiece paved the way for opera and the new Baroque style of writing. Like Beethoven's Eroica, 'Orfeo' meant that everything was changed forever.
3 Bach - Mass in B minor
Held up by many as the greatest work by arguably the greatest composer of all time. The B minor Mass has it all - wonderful lyricism, a perfect harmonic structure and some cracking fugues. Every composer since has had to have learnt something from Bach.
4 Beethoven - Symphony No 3 (Eroica)
This is the symphony that really ended the Classical era as it had been known up till then. The Eroica could be considered the first Romantic symphony. Its huge scale (it was the largest symphony to be composed at that time), structural and harmonic daring and audacity meant that nothing would ever be quite the same again.
5 Beethoven - Symphony No 9 (Choral)
The 'Big B's second entry - and the second of his 9 symphonies to make it into my list. By any standards, the Ninth is a towering masterpiece. 20 years after the 'Eroica', Beethoven again wrote the largest-scale symphony ever written. Some would regard it as still the greatest symphony of all time.
6 Wagner - Tristan und Isolde
This is where tonality received it first true challenge. The harmonic language in Tristan reached new chromatic heights. Wagner had also developed his 'through composed' style (as opposed to 'number operas' which had been the norm until then) and the use of the Leitmotiv, where characters and other ideas were associated with certain musical themes.
7 Stravinsky - The Rite of Spring
The last of Stravinsky's revolutionary ballet scores for Diaghilev, The Rite paved the way for a new kind of music where melody could be subserviant to rhythm, orchestral colour and sheer impact.