Now, this question tends to be quite vague, but it goes a long way if the audience member knows their personal likes and dislikes. The best movie of all time, in my opinion, would be Scarface. Now, this movie was, by the "blockbuster ratings" means, a disaster. It initially received very small profits and at the time was not given the cult classic status it has now.
Al Pacino may have been why I initially watched this movie but it was the script and storyline that kept my interest. Being a huge Pacino fan, I may have some bias, but if you follow the story you can tell that it's not just about a "wannabe" gangster who gets in way over his head. It begins with a small time hustler, Tony Montana, coming to America with the rest of the dredges Fidel Castro sent from Cuba. The American government places all the new Cubans in a Miami camp where they are fenced in and not allowed to leave. These people have gone from nothing to something so limited it may as well be nothing. The American Dream is not so prevalent, yet.
Tony has been portrayed as a dreamer already, and he begins to envision his potential. When riots break out and Tony escapes, he and his friend/right hand man Manny, try to see what jobs they can get. Being set in the 80's, the drug cartels in Miami target these disenchanted Cubans to do dangerous jobs for them at small prices. Tony doesn't like being seen as a small peon and the struggle for power starts to grow within him.
Tony immediately gets thrown into a job that takes his friend's life and almost his, as well. He was naive in this business but learned quickly that there is little trust amongst men and where there is money there is corruption. From then on Tony's life begins to spiral. He begins to want more than he needs and when his power begins to grow more and people begin to trust more in him, he starts to lose sense of his place. This all culminates with a drugged up, agitated, and stubborn Montana trying to fight off an army of Colombians trying to kill him.
His rise from diner busboy to cartel leader proves that the American Dream is alive an always will be. What I love about living in America, is that I can be whoever I want to be if I can envision it. It proves that having a calm and collected head will keep you afloat in your businesses. Hate, spite, and greed will only take you down a peg or two. Is it the best movie of all time? Maybe not to you, but it is an amazing script with a lot to take out of the dialogue and demeanor of Tony Montana.
Again, it is impossible to state what movie is the best movie of all time, but if the script is meaty, the dialogue is meaningful, and the acting hits the right personality, then it is a great movie. The Godfather, Citizen Kane, Scarface, or The Good, The Bad, and The Ugly. I can't rank any of those above the other, so I have to pick the one I enjoy the most.