Dungeon Keeper, for the list of reasons that have already been said.
Max Payne, for being amazing and not being max payne 2. plus, the kung fu mod!
Gears of War, it kinda feels like cheating to add this because the game was designed to be badass from the get go, rather than all the others which we decided were badass. still, you and a friend simultaneously kicking open doors on co-op, buddy movie style, is rediculously badass
flat out 2, or ultimate carnage or whatever the version i have is called. running people off the road who try to overtake you, only for them to crash, launch through the windscreen and land under your wheels just wins.
mount&blade has a whole genre of badassery to its self. i dont even need to explain.
the specialists for half life 1.
bloodrayne for the mental mincing up of nazi soldiers in the rage mode or whatever.
battlefield 1942, the best of the series. ramming an aircraft carrier into an enemy battleship, dropping bombs through house windows, buzzing the runway and clipping soldiers out to sea...
jedi knight 2, for the dismemberment. spawn 50 stormtroopers and run at them swinging. fun!
and finally, Total Annihilation. played it? then you know.