Some of my favorites:
CoD 4: Crawling through the tall grass and knifing everyone that walks by - like a land shark!
MW 2 (Before everyone and their mother had a shotgun): Went full on Ninja with marathon pro, lightweight pro and commando pro on the level - forget the name - with the shanty town with all the rooftop access. I was jumping from roof to roof, landing on people with my knife or hurling throwing knives at people. Even managed to knife someone through a window as I was jumping through it.
Battlefield 2: Think the level was called Backstab. Got air dropped onto an enemy capture point and managed to hold it myself from constant waves of enemy soldiers. Kept switching positions so they never knew where to look when coming up the stairs.
Metal Gear Online: Forget the level (horrible with names apparently) but the abbreviations were AA and it was the closed in, dark level. The enemy had our spawn locked down by snipers while soldiers routinely stormed the place to keep us pinned down. Through a combination of playing dead when enemies passed by, rolling through small openings and careful movements I snuck up behind the snipers and slit each and every throat before they knew what was goin on.
Also in MGO, took on Raiden hand-to-hand and won. It's kinda easy to get him in a CQC hold if you time it right but it's still damn awesome.