You know, I don't think it is possible for me to point to a single film and say "that is the best ever." There are so many different measures and perspectives to take into account, and so very many good films from all different kind of genres. Trying to identify the industry zenith is too much of a transcendental process for me. How can you compare Twelve Angry Men with Ringu? How can you measure Gladiator against One Flew Over The Cuckoos Nest?
It's all down to personal taste. But even personal taste can change. It's amazing how a movie that you thought was terrible ten years ago can seem much better when you sit down and watch it again.
I think it's easier to band or group movies. That is to say, these movies are the worst, these are bad, these are average, these are good, and these are exceptional.
Even this is hard. I think you would struggle to find any movie that everyone agreed should be in the exceptional band. But I do think that there are some movies which would be supported by broad consensus. The various polls that come and go, not least the IMDb, provide as good a guide as any, though I wouldn't go so far as to say that any of these were definitive.