Like with consoles, we like to count the units sold, not the revenue. I guess though, for managers/CEOs the revenue is more important.

Idk if US-only but in France for eg we count how many tickets were sold, so the most popular films are based on how many people went to see the movies.

And the 1st spot remains Titanic, with 21 million people.

Tbh, revenue numbers are funnier to follow than ticket sales 

Plus they are actually more important than ticket sales from a business standpoint 

Yeah, counting it that way may move the top spots, as cinema tickets tend to get more expensive. I also think the number of people that watched it is more important, alone as it says something about the cultural impact. How many people know this movie and it's scenes.

Why funnier ?

And yes indeed, from a business standpoint, but when it comes to records and "biggest" type of lists, revenue is less important from my perspective.

Avatar 2 is basically complete at this point, Avatar 3 was finishing filming late last year and Avatar 4 is being filmed now as well in New Zealand. I imagine most filming for Avatar 5, the last of the four sequels currently in production, will be complete too by the release date of Avatar 2 next year (it was supposed to release this year, but pandemic and all I guess).

This is more of a Lord of the Rings-esque superproduction than a "one sequel at a time" approach.

It's more exciting to follow box office because it makes the rankings, records and predictions changing almost every year

One thing that was depressing about following music sales is how the market was plummeting every year and record sales decrease fast as well

Boy, I cannot imagine how much of a gamble that is. Avatar was the hottest movie in 2009, but it's been more than a decade since then. If Avatar 2 doesn't capture the success of the first one, I can't imagine how much money lost we'd be talking about then.

BoxOfficeMojo's Top Lifetime Adjusted Grosses chart does give an idea of which movies were seen by more people: https://www.boxofficemojo.com/chart/top_lifetime_gross_adjusted/?adjust_gross_to=2020

Using that chart, Gone With the Wind is #1 by a comfortable margin and Avatar and Endgame fall down to #15 and #16 respectively.

Thanks, quite an interesting chart. Yes, it is estimated based on grosses and ticket prices at the time, but it should be the right ballpark. Now older movies are much more present. And what I said about cultural impact is more bound to how many watched a movie - that feels true about Gone with the wind. Albeit that old the cultural impact is still felt. Similar with the first Star Wars, E.T., Titanic and Jaws. Sound of Music and the Ten Commandments are more surprising though.