Both are fantastic actors! If you would have asked me 15 years ago I would have easily said Deniro. In that time though, I think the 4 or 5 movies that starred Lewis were very in-depth. He does drama, that's his thing as where Deniro does action, drama, and comedy as well.
Huge fan of Last of the Mohicans and Gangs of New York!!!
Day-Lewis is having a magnificent run since he came out of retirement, but Deniro still wins a close UD based on his activity.