* William Shakespeare: While he wrote many famous plays, his most famous poem is likely "Sonnet 18", also known as "Shall I compare thee to a summer's day?".
* William Wordsworth: His most famous poem is likely "I Wandered Lonely as a Cloud", also known as "Daffodils".
* William Blake: Known for his visionary poetry, "The Tyger" and "The Lamb" are often considered his most famous works.
* William Butler Yeats: Among his many well-known poems, "The Second Coming" and "Sailing to Byzantium" are often cited as his most famous.
Please specify which "William" you're interested in so I can provide a more accurate answer.