Before Master Chief and Cortana made themselves legendary, their revered predecessors kicked just as much ass.
Beware of spoilers
Basically everything Durandal ever does, but his best moments include:
Killing half of the Pfhor's best battlefleet with nothing more than his upgraded corvette "Boomer". He knew he was going to lose, had nowhere to run, so he decided to go out in style, and believes that this action will get him into the Pfhor's military history books.
Durandal: I'd have erased my seven times table to hear what the combat technicians on the Khfiva shouted when they learned I could focus a particle beam at nearly twice their maximum range.
Durandal faking his own death, then recreating himself on board the main Pfhor flagship, and sending Tycho's ship crashing into the moon. Then he caps this off by using a plasma beam to write an inscription celebrating his victory in the moon, visible from orbit... in Latin.
Ten-thousand years after Marathon II, he buzzes Earth in a Jjarro battleship... just to say "hi" and make sure humanity doesn't forget him.
A few of Security Officer's feats includes:
Rampaging solo on various Pfhor ships, particularly in the first game, where he practically starts a species-wide revolution while at it.
Humiliating the Pfhor's best Heavy-Armor unit just for the hell of it.
Jumping through time/alternate dimensions/whatever in order to stop the Eldritch Abomination from destroying the universe.
Meta: Doing all of above only with his FISTS when possible, Vidmaster style.
Action-packed Maps that also serve to empathize the badassness of the Security Officer:
Marathon: Try Again
Durandal: If I Had A Rocket Launcher, I'd Make Someone Pay and All Roads Lead To Sol
Infinity: You Think You're Big Time? You're Gonna Die Big Time!.