There is a website called "Box Office Mojo" that adjusts box office revenue for ticket price inflation. You can see their rankings at Box Office Mojo ALL TIME BOX OFFICE. "Gone With The Wind" is still the #1 movie all-time when you take the change in ticket prices into account. When GWTW first came out in 1939, prices were alot lower. The average ticket price was only $0.23. Now it is $7.46. You can read about how they make the adjustments and what prices were in the past at Box Office Mojo: ADJUSTING FOR TICKET PRICE INFLATION.
So the average ticket price is 32.44 times higher than in 1939 (7.46/.23 = 32.44). This actually far outstrips the overall increase in prices. The Consumer Price Index (CPI) was 13.9 in 1939 and it was 214.54 in 2009. So prices across the economy are 15.4 times higher (214.54/13.9 = 15.4). So movie prices have increased at about twice the rate of overall prices.
So we might want to cut GWTW's adjusted box office total of $1,507,252,900 in half. That would leave it a mere $753,626,450. That is still higher than Avatar's $561,317,300 (which only includes U.S. ticket sales but I assume the same is true for GWTW). But GWTW was making money for a long time after 1939, so some of its later box office revenue would have be adjusted downward as ticket prices rose. I just don't know how much of its revenue or ticket sales came early on. My guess is the vast majority. That is usually what happens with movies. They make most of their money early on.
But MOJO says:
"Some movies have been released several times over the decades, and we do account for this. For example, Snow White was released in 1937, but half of its lifetime gross is from re-releases in the 80s and 90s, so each of these releases is adjusted according to the year it earned its money."So that makes me think that GWTW is still the leader.
Update (1-30-2010): Carl Bialick has an article about this in the Wall Street Journal called What It Takes for a Movie to Be No. 1.