Grand Finales for me is defined by films that finish off either a trilogy, a franchise, or a specific selection of films.
Trilogy: The Lord of the Rings: The Return of the King
Franchise: Harry Potter and the Deathly Hallows: Part 2
Specific Selection of Films: Avengers: Endgame
It is important to note that if a franchise is split into different trilogies (The Lord of the Rings and The Hobbit), the final installment in each trilogy will be included. Films like Avengers: Endgame and Logan are also included. Although it isn't necessarily a finale to the franchise, it acts as a finale to the main cast of characters, therefore it is included.