Getting to the third phase of the Death Knight. There's something about seeing the Death Knight stumble backwards in slow motion that is just awesome.
Getting killed by the God King. He is just that badass.
The God King's defeat, where you impale him with his own sword.
The scene where Siris bangs on the bell to draw the God King to him in Infinity Blade 2. Even better if you strike the God King down.
The opening of Awakening, where Siris easily slices his way through the thugs of an official attempting to make a power grab in the aftermath of the God-King's death by telling the people in the town that his actions are endorsed by him. Siris slaughters all comers with the Infinity Blade and then makes the guy leave. Ruined afterwards since in order to avoid punishment from the Deathless the townspeople invite him back, but it's still ridiculously awesome.
The ending for III. At Isa's warning, Siris — currently held up and choked by Galath, who's ready to impale him with the Infinity Blade Mk. II, and surrounded by the flames of the Ark's thrusters — wrests the Blade away from him, and proceeds to impale him with it. At first, it looks like a repeat of the ending to Redemption, as Galath's just lying there, gloatingly asking Siris if he really thought a blade designed by him would be able to kill him. Siris, however, just pulls out the device Galath used to mindwipe Ausar, and has this to say:
Siris: I never thought it would kill you. But I do believe...in redemption.
Siris then plunges the Redeemer into the Infinity Blade's hilt over Galath's panicked protests, at which point Galath's QIP is driven away from his body to reincarnate without his memories. Cue end shot of Siris and Isa teleporting away while the Ark takes off without Galath.