Six out of 10 is ridiculously low, but indivdual tastes vary so much that it is probably impossible to hold any one movie up and get anything close to universal support as to it being "the best of all time." Still Citizen Kane is at or near the top of so many critics and avid movie goers lists that it has to be in the discussion, and no, I don't buy the theory that it is sort of a word of mouth consensus. I'd definitely put it in the top 10. The story is fascinating, the performances are superb, Wells' use of lighting and camera angles are so incredible that this is a fascinating film to watch even without sound. I don't think it is necessary for everyone
to view it as the best ever. But Kane is a heavyweight, and a groundbreaking film.
I'm a huge baseball fan, and part of the appeal of the game is it's ability to generate tons of interesting numbers and valid comparisons between players to participated in different eras. Baseball fans can argue endlessly about the merits of Babe Ruth vs. Hank Aaron or whether Whitey Ford had better command than Roy Halladay. This is the same kind of debate. There might be certain legitimate measures of comparison, but at the end of the day, it's your opinion against mine.
As to my own pick for best movie of all time, I have to cop out. I'm not sure how one could say that Gone With the Wind is in some way better than, say, City Lights...they are different films that evoke different emotions, different thoughts. There are a lot of films I like a lot for very different reasons...I'd have a difficult time really pinning down a top 10 let alone one single film