The "How Bad Can I Be?" song sequence. For those who condemn this film as a betrayal of the book's theme needs to see this, which is an infectiously tuneful skewering of the modern economy with its corporate excesses and the self-deluded rationalizations of the 1% as the Once-ler completely buries his conscience and wrecks the land with his greed, even co-opting the Lorax for his own ends when he tries to stop the rampage.
When O'Hare's goons turn on him, straps a rocket to his head, and launch him out of Thneedville.
Any time that Ted is on his scooter, it will eventually end up being awesome. Especially when he starts using the wacky "Dr. Seuss-style" buildings and their unusual structures to his advantage.
"My name is Ted Wiggins, and I speak for the trees!"
The Lorax's "epic" arrival.
In the climax of the movie, O'Hare has turned the entire town against Ted, Audrey, and Grammy Norma, attempting to stop them from planting the last seed. So, in order to get the townspeople on his side, Ted, a twelve-year old boy, jumps into an earthmover and smashes down the wall surrounding Thneedville, exposing the wasteland outside of town and revealing Thneedville as the Crapsaccharine World it really is.
On the commentary, the director reveals that an ending they almost went with had The Once-ler, of all people, smash down the wall with his Truffula Tree Axe-Hacker!
Everything Grammy Norma does. Nudging Ted into seeing the Once-ler, snowboarding, grabbing the seed during the chase scene, and giving O'Hare a whack on the head during the final musical number!
The "Let it Grow" number.
Ted's mother chewing out O'Hare, and later distracting one of O'Hare's bodyguards during the chase scene DESPITE her immense fear of him.
More subtly, the Lorax's departure. The Once-ler, never having witnessed the Lorax's arrival, accuses him of being a fraud with no actual magical powers. Then the last Truffula tree is felled, and a magical aura surrounds the Lorax as he levitates into the air wistfully, as though saying, "You want magic powers? Here's that magic power, and it's there to tell you how much you suck!"