The first two-thirds of Puss' flashback to his childhood, from Imelda taking him in to his becoming Blood Brothers with fellow outcast Humpty Dumpty.
Much of Puss and Imelda's interactions, especially when he calls her "Mama" for the first time.
Humpty's Heroic Sacrifice, and the revelation that he was a golden egg himself all along. And then the geese take him home with them. Aww.
Puss and Kitty's relationship on a whole is definitely this; just the way they click together so well, and are so perfectly at ease with each other is so heart-warming sans the time Puss realised she was actually working with Humpty, but she redeems herself by pulling a Big Damn Heroes moment.