3. Avengers 5
A leading contender for Avengers: Endgame's box office championship would have to be the sequel. The historic trend of Avengers movies increasing at the box office over their predecessor, with the exception of Avengers: Age of Ultron, suggests that Avengers 5 has a strong chance to become the new global box office leader.
Avengers 5 has no official release date as of yet but Kevin Feige, president of Marvel Studios, has indicated that plans are underway for a new Avengers film featuring new stories and new characters.
A long wait for the next Avengers movie will surely only build anticipation further potentially increasing box office results. More time passing could also help Avengers 5's box office as growing markets such as South Korea and Brazil continue to grow, offering a chance for Avengers 5 to gross more in these markets than Endgame.
However, Avengers 5 may not achieve the lofty goal of defeating Endgame at the box office, because it will be hard for Marvel to recreate the astronomical event movie feeling of Endgame in time for Avengers 5, but as shown by Endgame defeating Avatar if anyone can Marvel can.