Assassinating Majd Addin, if only for the combination of the moral unambiguity of killing the godless, sadistic bastard and Altair's glib ending line:
Addin: "You would have done the same in my place."
Alta´r: "Long ago, perhaps. But then I learned what happens to those who elevate themselves above others."
Addin: "What is that?"
Alta´r: "Here. Let me show you." (stabs Addin in the neck)
Altair wading through the battle of Arsuf, leaving a carpet of bodies on both sides. As this is the endgame, you'll probably be so used to the combat system that you'll be slaughtering anyone that attacks you. In regular combat, when you kill several guards in a row without getting hit once it can cause the other guards to panic and run away, presumably to go change out of their armor.
Not only Altair going through dozens of soldiers, especially if the player is REALLY good at swordfighting and kills them all without taking a scratch, which, according to the synchronization system, is exactly what Altair actually did.
Malik, from the second he has his Big Damn Heroes entrance near the end of the game, and promptly follows it up by telling Alta´r that he'll do his best to hold off a good chunk of Masyaf's assassin population so Alta´r will have a clear path to Al Mualim. What's even better is that Malik does it. He holds off a veritable Zerg Rush of Brainwashed and Crazy assassins with only one arm and four guys, and shows up after Al Mualim's defeat without a scratch.
Apperently this one often causes people to forget that he also had one at the very start of the game. After Alta´r is cut off from Malik and his brother, you are supposed to assume that they get slaughtered in the other room, being outnumbered by elite Templers and having apparently no way to escape. Jump forward to several minutes later when Malik shows up not only alive (albeit with his arm so wounded that it needs to be amputated) but with the golden Apple as well.
Alta´r's fight against Al Mualim, in which he "pierces the illusion" and proves himself to be resistant to the Piece of Eden's effects.
Alta´r again, when he calls Al Mualim out on his bullshit twice: The first time being partway through the game, where Alta´r demands answers and snarls that he is not a dog and will bow to no one even though he knows Al Mualim might kill him for it; and the second being just before (and somewhat during) the climatic battle between them, over the course of which Alta´r delivers some beautiful Shut Up, Hannibal!/World Of Cardboard lines.
"Truth be told, Master... I didn't. I took a leap of faith."
Here's another excerpt:
al Mualim: "I have offered you a chance to restore your lost honor."
Alta´r: "Not 'lost,' taken by you, and you set me to fetch it again like some damn dog! You said the answer to my question will arise when I no longer need to ask it, so I will not ask. I demand to know what binds these men!"
King Richard gets one by offering Altair a chance to fight for his life, something he had no obligation to do. He quickly surmised that Robert was capable of treachery, and allowed Altair to eliminate him and his fellow Templars.
The Vigilantes get one every time they step in to help Alta´r. They get an especially big one whenever they grab hold of Alta´r's fleeing target and very thoughtfully keep him in one place until Alta´r catches up to stab him.
This is possible with several of the targets, but it's most likely to occur with Talal since his escape route runs directly into a particular Vigilante spot.
The fact that they call you 'friend' while holding your target makes me wish the game had a brofist command.
The first time you successfully evade all of the high-alert guards in order to get right up to your assassination target, and kill him without being noticed.
Anyone who has done the following gets one of these: You have never played the game before. You get to the part where you lose all your stuff. All you have is your fists and four tiny little bars. What do you do? Punch every guard you see to death, and (mostly) survive.
An understated one happens in Bloodlines when Templars corner Alta´r on top of a building. In response, the Assassin faces his enemies, holds out his hands, and calmly falls of the edge in the series' only leap of faith done backward.