Absolute greatest? That's a tough one. I'd have to turn to the big epics of the '50s and '60s and '70s. The Ten Commandments comes to mind. Although I saw it again after many years, and I realize it's really over the top. In the same vein, Spartacus, Ben Hur, and Planet of the Apes (the original).
I'm sorry but all the latest big blockbusters just don't stack up for me against the classics with big scenery, often armies of extras, and special effects done with God knows what.