One of his most highly regarded novels is 'The Old Man and the Sea'. It's a short yet powerful tale of an old fisherman's struggle against the sea and a giant marlin. The simple yet profound narrative showcases Hemingway's unique writing style.
In my opinion, 'For Whom the Bell Tolls' is also a strong contender for his best novel. It delves deep into the Spanish Civil War, with themes of heroism, sacrifice, and the human condition. The characters are complex and the story is filled with action and moral dilemmas.