This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Walk the Line
When Johnny interrupts the concert to ask June to marry him.
Johnny: I'm asking you to marry me. I love you, June. Now I know I said and done a lotta things, that I hurt you, but I promise, I'll never do that again. I only want to take care of you. I will not leave you like that dutch boy with your finger in the dam. You're my best friend. Marry me.
The scene where June, the free-spirited, strong woman, can't bear Johnny's addiction, she silently cries in her car, then hides the pain when she sees her daughters
The scene where she's at a store, and a woman chastises her for being divorced. June just says, "Sorry I disappointed you". Before, she was happy and getting along with one of the workers there. That change in emotion was heartbreaking and subtle. Forget the haters, Reese Witherspoon is a fantastic actress and 100% deserved that Oscar.
The scene where Johnny has gone through detox and June gives him a bowl of raspberries and he weeps and hugs her. Brilliant acting from Phoenix and Witherspoon.