The Last Supper by Leonardo da Vinci ; at the Chapel of Santa Maria del Grazie
Leonardo da Vinci's last Supper was and is one of da Vinci's 2 most famous works, the other being the "Mona Lisa" situated at the Louvre in Paris.
This painting was not done in the 'fresco' or wet painting style so common of the 15th century but (pardon my ignorance) but rather it was done in a more painstaking tempera and oil on gypsum before mounting it to the wall. The painting is kept in a climate control room whereby the humidity is kept purposely low to prevent moisture from causing damage to the wall (fungus etc.)
If you do go to Milan, do book this tour early as the number of people allowed to view the Last Supper is very much kept to minimal so as not to cause damage to the wall.
The painting depicts Jesus of Nazareth seated at the centre and having the last supper with his 12 disciples before he is betrayed by Judas and even his own dearest Peter.
Michelangelo's David in the Accademia de Belle Arti (Fine Arts Academy) in Florence.
Another must visit site is Michelangelo's David (of David and Goliath story). This 6 meter marvel of engineering and sculpting brilliance is a must see by anyone who visits Italy. Michelangelo himself said that he saw the people inside the rectangular blocks of marble and his job was to free them from the blocks.
There are numerous blocks showing semi completed works where the people seem to be struggling to get free from inside the marbe block.
Such was his genius which still resonates till today.
Accamedemia in Florence.
Carpe Diem.