What is the masterpiece in "The Last Leaf" by O. Henry?

In "the Last Leaf," the masterpiece is essentially Behrman's final, grand artistic endeavor. In the text, we learn the old painter, despite being a failed artist, has always envisioned creating a piece of art that will distinguish him as a noted practitioner of his craft. He never quite succeeds in fulfilling his goal, however, and as time progresses, he becomes a distinctly cantankerous and ill-tempered fellow.
Behrman is not a man without heart, though. When he learns his neighbor, Johnsy, is dying, Behrman sets out to paint his life's masterpiece. Johnsy is depressed and suffers from a deep malaise that is as physical as it is psychological. She believes she won't live out the remainder of her life and is content to die as soon as possible. Johnsy tells her companion, Sue, that when the last leaf on the ivy vine falls off, she'll be ready to die. Sue doesn't want Johnsy to continue in this morbid state of mind. She pulls down the blinds of the window in Johnsy's room, but knows this is only a temporary fix.
When Johnsy demands to have a look at the ivy vine, Sue has to humor her. When Sue pulls up the blinds, both are surprised to see the last leaf still hanging on the vine. In due time, the last leaf inspires Johnsy to snap out of her depression. She decides to live and to live as fully as she can. It is only later when Sue discovers the truth that both women realize what a true masterpiece Behrman had really painted. When the last leaf fell off the ivy vine, Behrman had braved the cold rain and snow to paint a replacement leaf on the vine. Because of his selfless action, Behrman contracts pneumonia, which eventually costs him his life. Yet, his very realistic portrayal of a leaf is responsible for saving a young woman's life.
Essentially, Behrman finally realizes his dream of creating a real masterpiece, and his final work is a testament to the greatness of his artistry.