He took down an army consisting of around a hundred thousand Covenant aliens on his own.
Destroyed a huge fleet consisting of around a hundred Covenant ships on his own.
He crash-landed on Earth and survived.
He escaped an exploding planet.
He escaped a planet full off the strongest Alien race in the galaxy, and also managed to help out a few people stuck on the planet.
That's called "Thursday," on his calendar, lol. And I'll have to agree; Master Chief is epic, but I think the most powerful ever is...*drum roll*...ANYTHING IN GARRY'S MOD. Seriously, you can build ANYTHING in there that can be more powerful than other stuff:
Rocket Powered Flying Toilet, with built in Flamethrowers? Check
Whatever Honeydew can think of in the Yogcast's Robo Sumo Challenge? Check
Gigantic crab-robot-thing? Check
Rocket Powered Toilet race car? Check
Combo of all games purchased on Steam? Check
I could keep going on forever with this list, because...THE LIST TO BUILD STUFF IN GARRY'S MOD IS ENDLESS. You can even make your own maps/games in the game for others to play, I think. Should I continue?