The E3 trailer is one. Ezio is able to approach his target easily while most of the guards are taken out by unseen allies, the rest effortlessly killed by Ezio himself. Just as he's surrounded, a storm of arrows fall upon them, and he promptly counters two charging horsemen like they're nothing - then his target summons the Papal Guard. Ezio not only isn't worried by this, but four Assassin Recruits join him, all prepared to kick ass and take names - the trailer ends just as these two forces clash. And this isn't adding in the music that amps up it's awesome rating considerably.
With few exceptions, everything seen in the trailer is perfectly doable in single-player campaign. note The main exception being the final cool-down between Arrow Storm and summoning recruits.
At the end of Sequence 5 Claudia's girls have stolen a chest full of loot from the Templars, but the Templars have tracked them back to the Rosa in Fiore. Ezio enters expecting a big epic fight...and finds his little sister Claudia standing over four dead guards with a knife in her hand. The respect and admiration in Ezio's voice also doubles as a Crowning Moment of Heartwarming.
Apparently this little sequence is crowning in so many ways to warrant it on Funny, Heartwarming and Awesome.
After Cesare murders Rodrigo and flees to retrieve the Apple, Ezio still stops to close Rodrigo's eyes and utter yet another "Requiescat in Pace." It just shows how far beyond the conflict Ezio's gotten when he shows such respect to one of his worst enemies.
Ezio using the Apple to make Cesare's Papal Guards kill each other.
The Mercenary arena battles, it really shows how competent Ezio is and how widely he is respected. When he is shirtless, surrounded by mercenaries half his age, twice size and outnumbered 4 to 1 what is the challenge? to survive? to take at least one of them out? to win? No. The challenge is to see if he can do it in under 35 seconds.
To add to the awesomeness of this, the "full sync" challenges are implied to have actually happened the first time around. So yeah, He knocked out four men, all of whom closer to their prime than he, in under 35 seconds flat.
Leonardo's machines missions.
The first one is Riding a cannon which you stole from the Borgia's army through the countryside, with soldiers on horseback on your tail whilst Ezio fires the cannon at them as he rides back to Roma? HOW CAN THAT NOT BE AWESOME.
You get to use LEONARDO'S TANK to kill scores of soldiers, cannon emplacements and TWO other tanks. Best mission of the whole game.
Even acknowledged in-universe as awesome. Right before Ezio destroys the tank he says he wishes he could keep it instead since it was just that awesome and the player probably agrees.
Made even more awesome by the implications of doing it at full sync, in which you have to avoid being hit at all. This means that the actual Ezio did all of this without getting hit once.
The flying machine mission, in light of the second game. In that one you needed fires set around the city in order to provide the glider with lift when it started to lose altitude. The only way to fight the guards shooting at you was to descend enough to kick them in the head. This new one, however, can launch fireballs down on your enemies...and the fires provide the necessary lift! So in the process of turning his invention into a machine of death, Leonardo also made its flight self-sustainable...via the exact same "unforgivable" act of making it a weapon in the first place. Only Leonardo da Vinci could turn Insane Troll Logic into a moment of awesome.
One of Ezio's last lines before he kills Cesare
Ezio: (sounding a lot angrier than usual) ''"Che nessuno ricordi il tuo nome. (May no one remember your name.)"
The DLC, in which Leonardo snarks at the people who have him tied up and at their mercy and keep beating him up for information.
Monk: "Where is the entrance?"
Leonardo: "Why? Do you not know where it is?"
Monk: "How could I?"
Leonardo: "You know mankind's wants and needs; I assumed discovering the location of a mere hidden entrance would be an easy feat."
Monk: "...Why, you.... WHERE IS IT?
Hearing the vigilantes taunting the guards about the various ways they will die at your hands. Bonus points for proving the vigilantes right mere seconds later.
The random two Assassin recruits who each throw a knife at Pietro's bounds on the crucifix. Neither of them even waited for him to hit the ground before they turned around to fight off some guards for Ezio, they were just that confident in their aim.
The point where Ezio wields the Apple of Eden. Summon all of the assassin recruits. They (along with Machiavelli, Claudia, and La Volpe) might be able to take on all of the Borgia forces by themselves. Thus it is possible for Ezio to stand there holding the Piece of Eden WITHOUT BEING ATTACKED.
The Malfatto mission requires that you kill him using your recruits for full synch. One of the Assassins recruits can pop out of a bale of hay and stab him point blank. They are EVERYWHERE.
Really, any time one of your little recruits goes above and beyond the call of duty (and it's usually hilarious, too). Signaling one recruit to eliminate one inconvenient guard only to have him or her take out the guard, his four friends over in the corner, the target, and the target's dog is not only awesome (and a testament to Ezio's training aptitude) but hilarious.
Speaking of the recruits, any time one of them reaches max level and Ezio initiates them into the order. The initiation ceremony is pretty cool, and at the end of it, the Assassin will now sport a Badass Long Robe much like Ezio's.
It's always kickass when you've got a full Assassin meter: you can just hold down a button, and whatever enemy you've got selected is killed in a hail of arrows.
Assassin Recruit Contracts; when the Rome Guild is just starting, there are many that have to be dismissed out-of-hand because they would be suicide for the fledgling Assassins. By the end game, Ezio has a full roster of Master Assassins. Just four of them can reach 90-100% chance of success for even the most difficult of missions; this is still one short of a full team.
Killing a guard with the heavy weapon special attack, especially from a long distance. Holding down the attack button for a melee weapon prepares that weapon's special attack (throwing dust, using the gun, or throwing knives, for instance). The special attack for heavy weapons? Throwing the weapon itself. There's even a DLC achievement for hitting a guard from a long distance away.
Just how damn much Ezio has grown since his early days. From the Borgia/Templar perspective, it is a From Nobody to Nightmare situation.