For me, coming out victorious against all odds is the most satisfying way to kill.
For instance, when outnumbered 3 to 1 in Gears of War, all of them advancing on your position, and you somehow oneshot all of them, one after another, in a glorious display of grace and gore, with just a sliver of health left.
Or in Team Fortress 2, with all of your other teammates dead, you and a medic buddy somehow hold off an entire teams onslaught as they try desperately to push the cart two feet into the final checkpoint, leaving a pile of burning/shredded/gibbed BLUs strewn about your base.
Or Left 4 Dead, the enemy team (with full health and medpacks) is about to make it to the helicopter, and you are the last tank. After an incredibly hard battle with a noob team backing you up, you somehow finish them off by knocking one or two off the building, then cornering the rest as your team finally helps out and downs them all at the same time.
All of the above leave you with an adrenaline rush and a feeling of pure awesome that can only be described as "HELL YEAH!"