Optimus erupting from his hiding place in Cade's barn when Savoy prepares to kill Tessa. "HERE I AM!"
Even in his weakened and damaged state, Optimus still manages to put up a fight and hand Lockdown a fairly humiliating takedown as he is pursued across the factory rooftops, attaching a crane hook to his chest and then throwing him off the roof.
Optimus Prime does not actually lose a single one-on-one fight in the movie. In fact, he's only beaten through outside intervention, and his tendency to help humans instead of finishing off nearly-defeated foes. Of course, the only one with the speed and pragmatism to employ this is...
And he manages to take out Optimus Prime himself with it. You read that right - he takes down the leader of the Autobot forces, who has dealt with several Decepticons much larger than him at once, with one well-placed shot. Though, well, whenever he pulls that trick off, Optimus is usually occupied with beating the shit out of Decepticons, severely wounded, or both.
Bumblebee tries to help Optimus out during his climactic fight with Lockdown. Lockdown quickly puts him in his place.
With Lockdown, there's even the implication that he beat Grimlock and the Dinobots. He says he's captured all of the Knights' Terminus.
Grimlock charges Optimus, only to be taken down in one backhanded punch.
From the fight in Chicago, one of Crosshairs' fighting scenes. He gives a swift crash course on using the weapons of the ship he, Bumblebee and the humans have hijacked to the humans, then front-flips off the back of it and deploys parachutes at the end of the somersault, firing at the pursuing enemy ships with precision while still in momentum to buy them time. Not only is he enjoying it, the camera makes sure to give his accurate shots - and his billowing longcoat - a full view and some slow-mo in the process.
Made even better by the dialogue:
Cade: Where are you going?
Crosshairs: ...To lay some hate.
Hound's fight against the many Vehicons in Hong Kong. He keeps them at bay with his arsenal of weapons, even while pointing out he's running out of ammo, guns and ideas. It culminates in him firing his bullet-cigar into the face of a Vehicon as they struggle against each other. He wins the fight.
Of special note, the fact he kills one just by chucking his helmet at him!
Also during the Chicago fight, Bumblebee fires grapples into a boat, and then yanks it up into the air and slams it into a Vehicon.
The fact that Frank Welker, finally voices Megatron/Galvatron in this film.
His first words being a Badass Boast to Optimus during their fight scene revealing that far from being KSI's mindless drone, the Megatron of old is back and more dangerous than ever
Optimus: You have no soul!
Galvatron: ...That is why I have no fear!
Cade, Tessa and Shane pulling their weight as a united team at the end after being in conflict for nearly the whole film to help Optimus Prime take down Lockdown with the creative use of one alien weapon and a tow truck. Special mention to Cade for having the guts to go one-on-one with Lockdown to distract him.
Shane's driving can veer into this. Driving backwards beneath what basically amounts to a giant magnet dropping ships, cars and debris in the narrow streets of Hong Kong and coming out completely unscathed? That's just one scene.
The Autobots rescue Cade from the KSI board room by having Drift transform into a helicopter, carry Bumblebee up several floors while inside the building before having the latter smash his way into the room that Cade's being held in.
Su proving her worth by kicking the asses of the two goons that nearly take Joshua and the Seed.
The random elevator guy soundly defeating the goon who was trying to get at Joshua and the Seed with Rapid-Fire Fisticuffs.
Cade pushing Savoy out the window
Cade's fight with Savoy is awesome; he hops rooftops, jumps, swings and falls down the side of a Hong Kong apartment building. If you've ever seen what those look like, that's nothing to sneeze at. All while Savoy is gunning for him, too.
During the final battle, Optimus saves Cade's life twice.
Optimus saves Cade from Lockdown by impaling the bounty hunter in the spark with his sword then slicing up through his head
Pretty much every shot of the Dinobots in the final battle.
Bumblebee vs. Stinger on the back of Strafe.
Optimus' message to The Creators: "I'm coming for you."
The escape from KSI, which has the Autobots racing away down the freeway with Galvatron and Stinger in pursuit. It culminates in a brutal brawl between Optimus and Galvatron.
When Optimus tries to get the Dinobots to help them he tells them they either stand with him or against him. Cue Optimus vs. Grimlock.
The Autobot reunion scene in the desert, especially with the swelling and extremely rousing music score.
Optimus scanning a new truck mode, complete with the Autobot logo rising out of the grill with the classic transformation sound.
Hell, ALL of the G1/Animated influences, showing that the movies aren't afraid to come back to their roots.
Just the fact that Drift is a Triple Changer. The movies work pretty hard to make sure that vehicle and robot modes are congruent with each other, so it's pretty amazing to think of how much planning went into Drift's transformation scheme and design, especially considering that they had to take his samurai motif into account.
Most of what Su Yueming does after Joyce's Heel-Face Turn, from beating up goons to driving a motorbike through the intensely busy streets of Beijing to get away from Attlinger with the seed.