The sentence should be hyphenated thus: 'It is the highest-grossing movie of the year.' The adjective is 'highest-grossing' - the superlative form of 'high-grossing'.
As of May 01st 2018, Black Panther is the highest grossing superhero movie in America [& the third highest grossing movie ever]Worldwide box office, The Avengers is the highest grossing superhero movie followed by Avengers Age of Ultron & Black Panther.
James Cameron's Avatar is the highest grossing movie worldwide w/ $2,787,965,087.
Avatar is the highest grossing Oscar winning movie of all time worldwide.Avatar is also the highest grossing Oscar winner domestically because the highest grossing movie, Star Wars: The Force Awakens did not win in the 5 Oscar categories in was nominated for.When adjusted for inflation, Gone with the Wind in the highest grossing Oscar winning movie.-If you mean the highest grossing movie that won the Best Picture Oscar, that would also be Gone with the Windunadjusted for inflation. It made 199 million dollars.
Australia
Titanic
The highest grossing Australian movie is Crocodile Dundee, with box office grosses over $47 million in 1986.
Most popular: Horror Highest Grossing: adults
Harry Potter
The Lion King :)
By highest selling, I assume you mean highest grossing. Highest Grossing Movie in the U.S.A (not adjusted for inflation): Titanic (1997) -- $600,779,824 (not adjusted) or $908,688,900 (adjusted) Highest Grossing Movie in the U.S.A (adjusted for inflation): Gone with the Wind -- $198,676,459 (not adjusted) or $1,430,476,000 (adjusted)
The highest grossing Australian movie of all time is Crocodile Dundee, with $44.7 million.