Avenger's End Game really only has Star Wars 9 as a top-grossing film competitor for 2019. However, Godzilla King of Monsters is the only big budget movie in 2019 that is truly new and awe-inspiring.
With Avengers EG you get literally nothing new. We get the relatively obvious conclusion with the same characters and abilities we've seen for over a decade. Even Captain Marvel will be old news by the time this movie hits theaters. The simple fact is that while it will be a great movie, it will be another super hero trope.
Godzilla King of Monsters is something truly new. For the first time ever we'll have a fully modern FX Godzilla + Rodan + Mothra + King Ghidorah + the possibility of a few other monsters. While these have been shown before in older movies and they have fought before, this is truly a new and unique story. This is truly a first for the super-modern iterations with new and fascinating takes on their personas and abilities.
This is the truly big movie of 2019. Godzilla of 2014 was the Return of the King, 2019 is the King's Reign!