Anton Ego's climactic speech. It's moments like this where Pixar really shows what it's made of.
All that is good in the world? The stuff dreams are made of?
The chocolate from our childhoods.
^ That right there was a crowning moment of TV Tropes heartwarming.
The quiet little moment immediately beforehand when Ego tastes the ratatouille and flashes back to his mother's cooking. When you see him for the first time, he's this cold, snide, hard, and impossible to please boogie man. Under all of that though? Buried deep, he was someone who loved food. Someone who once lived to experience, to taste and explore. He had the same fire that Remy has, but after years of criticizing, of making food a job, everything he enjoyed was naught but ashes in his mouth. It took something simple, just a simple peasant dish, something he'd probably not tasted in DECADES, to rekindle his heart. All the fancy glitz in the world couldn't do that.
The ending. Remy is the chef of his own restaurant, La Ratatouille, with Linguini and Colette. Blame the amazing French music.
Ego's grand finale line: SURPRISE ME!, because, man, weren't we all.
Also, Remy mentions how happy Ego is because after losing his job as a critic he became a small-business investor. Ego was the one who used his wealth to help Linguini and Colette with the finances they needed to start their own restaurant just for the sake of being able to continue to eat Remy's cooking and share it with the world. Think about that kind of act from him and how much it says about his love and respect of Remy's talent. It's probably one of the most win-win endings in a film ever! Happy tears.
Linguini revealing the truth about Remy being the actual cook and trying to rally his crew around that crazy notion. They instead walk out on him, including Colette, who also makes a motion to slap him before dropping her hand in tears and also exiting, leaving Linguini and Remy all alone in the kitchen.
"Get out of here! You and all your rat buddies! And don't come back or I'll treat you the way restaurants are supposed to treat pests!"