One of my favorite less familiar Peter O’Toole performances is as the unhinged Earl of Gurney in The Ruling Class. The movie starts out light and then gets crazily darker and darker, but it is the first part with several surreal musical comedy numbers that I liked the most. Unfortunately, I could only find the below blurry youtube video that gives a weak taste of them. (I should also mention that the Earl is totally bonkers and, in the first part of the movie thinks he is Jesus and later on, Jack the Ripper.)
Another is his performance as god-like director Eli Cross in The Stunt Man.
And then there is always and forever the glorious Lawrence of Arabia.