Every time I see a "largest-grossing" banner like this, I think, "Well, so what. Movie ticket prices have increased as the value of the dollar has decreased over time. The gross, therefore, will have to be larger, so it's another Meaningless Statistic(TM)."
But it's not like you can't measure the number of tickets sold. I mean, each box office will count the number of sold tickets, just for it's own sake. So why not measure the "best film" by number of tickets sold?
Granted, there's more theaters now than there were 10, 20, or 50 years ago -- but there's also more people in the US. If you're really a fanatic, get number of tickets divided by population at the time. That's at least more objective than comparing a movie that made $1 million in 1950, and a movie that made $100 million in 2000.