The moment near the beginning where John lunges at the Union troops who've come to collect him...and is instantly knocked out. When he wakes up in a chair in front of a Colonel, John lunges at the officer, punching his would-be captors and...Smash Cut to John back in the chair, now handcuffed. The Colonel starts talking, and John leaps up again!Smash Cut to John being thrown into a holding cell.
Even funnier? After the second Smash Cut, the Colonel's nose is bandaged.
The scene where Kantos Kan makes John take him hostage was hysterical.
The best part is just as he enters the room John's being held in. First he slams the door into two guards and strolls in all nonchalantly and says.
Kantos Kan: Ah Zadanga, where the men are as limited as the menu. And the woman, are as hard as the beds.
When John leads the Tharks to Zodanga to stop the wedding and one of the guards tells them the wedding is in Helium. Tars Tarkas then gives John a Dope Slap.
Becomes even funnier when you realize that with this scene, Andrew Stanton actually managed to have a John Carter of Mars film that includes the sentiment that, basically, their princess is in another castle.
Tars Tarkas is just hilarious in the whole climax. What does he say when his people have flown to the rescue as Big Damn Heroes despite their professed aversion to flying? Not some ancient battlecry, or order to attack, just, "Thank the goddess that's over with!"
Tars Tarkas almost attacks Tardos Mors and Kantos Kan, then realizes he's fighting on their side, so... he salutes them (having learned the gesture from Carter). Stunned, they salute back. Then Tars lets out a battle cry and throws himself back into the fighting.
When John, in the heat of battle on the flying ships, tries to leap to his feet after being knocked down and inadvertently leaps into the air and straight into the ceiling.
John and Tars Tarkas' first reaction to each other:
John: What the-
Tars: (in Martian) Hell are you?
John Carter is thrown into a cell with a badly beaten Tars Tarkas, who's too weak even to stand but comforts himself that at least his daughter is in the afterlife with her mother as John promised. John then hesitantly explains that his daughter is alive and also a prisoner — an enraged Tars Tarkas gets to his feet and starts throttling John. Then collapses on top of him from over-exertion.
John and Tas Tarkas in the arena:
John: Give me your hand! You got four hands, just give me one!