That bit on Assault on the Control Room where you first get outside into that big firefight with the marines and wraiths and shit then proceed to stomp everything with a tank
Scarabs as a general thing in Halo 3
Bungie showing the first seeds of destiny in Halo 2's "boss battles"
Prepare to Drop
Monsters- AKA the first in-game accurate portrayal if Spartan II's kicking arse
That bit where there's the big battle on Rea- sorry, got mixed up with the book for a minute there
Didact awakening. That scene when he's first introduced still gives me chills
"I regret everything" marking the regression of grunts back into little more than comedy figures for the series