1. The Trio Meets On The Train - Philosopher's Stone
This is the beginning of it all. Harry did a lot of stuff on his own, but without his two best friends, he probably would have died in the first couple years at Hogwarts. As he's sitting on the train, nervous but excited about what his future holds, a young ginger boy walks in and asks to sit down. Harry is so unused to having kids his own age actually interested in being friends with him that he eagerly accepts Ron's company. This is the audience's first glimpse of the boy who will become one of Harry's best friend, and his character is developed right in that first scene: he's very friendly, a little poor, and constantly feels overlooked by his older brothers. It doesn't take long before Hermione shows her face, a bossy little girl who, unlike Ron, doesn't immediately endear herself to Harry. Their acting is unpolished and immature, but regardless it's a fantastic introduction to the characters who will become the glue that holds the story together.