Lawrence of Arabia (1962)
Let’s get the obvious one out of the way. Lawrence of Arabia is a masterpiece, one of the best and most influential movies of all time. O’Toole stars as T.E. Lawrence, the British archaeologist and military officer who, during WW1, ends up joining the Arab Revolt against the Ottoman Empire. It is a historical drama/epic, inspired by films like Ben-Hur and The Searchers, and was also O’Toole’s first real introduction to audiences. He had come up on the London stage prior and had a few small supporting roles before this. Despite that, O’Toole was able to do a lot with the role.
This is as far away as you can get from the bland, personality-less leading man roles in action films today. Lawrence was often described as more of a poet than a fighter, and O’Toole brings his inner conflicts, impulsiveness and yearning through in the role.