My list of favorite actors is quite extensive, so it's hard to pick just one. But when it comes to the wonderful combo "favorite actor + favorite movie", then I will probably focus on "The Shining")
There was a lot of controversy around this film, in particular, Stephen King did not like it. Not only for the interpretation of his novel, but also for the selection of actors for the main roles. Stephen King did not see in Jack Nicholson the image of the naive and level-headed Jack Torrance. However, Stanley Kubrick's choice is closest to me.
My poster
Every time I re-read this novel, I imagined the main character exactly as Jack Nicholson managed to play him. This is by no means a naive teacher who suddenly began to go crazy - this is a complex character, within whom a real storm unfolds. His character touches upon rather philosophical topics: themes of the creative path, the search for oneself and the problem of loneliness.
Jack Torrance, "The Shining"
However, the inability to overcome the creative crisis leads Jack Torrance to insanity and an obsessive thought to destroy all the obstacles that do not exist in reality, which he saw in his wife and son. There is no longer a line between his fantasy and reality. An attempt to invade this world leads to its own death. His own mind and dream take him further and further away from people. The peak of this eerie transformation is the scene in the frozen maze, when Jack fell into the snow, and rising again, uttered a brutal cry that no longer has anything to do with a human cry, for he himself has long ceased to be such.