Al Pacino vs. Sidney Lumet...
This is very, very hard. I think it's safe to say Pacino is more consistent than Lumet is, but Lumet brings out the best of Al (Dog Day Afternoon, Serpico).
I guess when it comes down to it, I'd have to say Sidney Lumet. The fact that his career spans over fifty years, and three years after winning a lifetime achievement award he directed one of his greatest films, says it all.
BUT, I prefer Al.