Must have been asked before but interested to see what people would say. For me it is probably The Moon and Sixpence by Somerset Maugham, it was one of the first classic novels that I read, found it absolutely riveting and I enjoyed it so much that it really spurred me on to read a lot more books of similar subject and from around that period.
An honorable mention would be Butcher's Crossing by John Williams, the most visceral and powerful novel I've read where I felt that I really connected with the main character. The first novel that I finished and was really blown away emotionally by the end.
by aIltimers