Cast Away
I'm not saying Tom Hanks might have delivered one of the greatest performances ever witnessed by the human eye in this movie, but I am.
If it doesn't seem like much to you, imagine 2 hours out of a 2 hour and 23 minute movie having only you in it. Just imagine that. No other human to interact with, just yourself doing god knows what the directors ask you to do. Then come back and tell me how much of a masterpiece this was.