When dealing with the biggest films over time, one has to make a vital distinction first. One metric is admissions, which focuses on the raw quantity of tickets sold. The second metric, is the real value generated by a film at the box-office in terms of constant currency (usually USD)
This is a very important distinction, because the real value of tickets has varied between both time and between films released at the same time.
One elementary error I find on BO websites, is the direct translation of admissions to the current average ticket price. This is highly misleading, because it blindly assumes that any movie from the past would have been able to sell the same number of tickets regardless of the real price.
To give a real world example, ticket prices in the 90s were much lower in real terms than they are today. Adjusting the grosses of Jurassic Park, The Lion King, Forrest Gump, ID4, Titanic etc based on admissions achieved then and subsequently multiplied by the current (higher in real terms) average ticket lies on the misguided assumption that these films would have managed to sell the exact same number of tickets even at higher prices. The result is of course an inflated adjusted number that exaggerates the success of films from previous eras.