Oh man I have a ton, but I have 3 three that really stick out.
I was playing with my little brother, we were playing Borderlands, we were about level 14 and the guys we were pinned down by were level 16 and 17.
Me: Oh god D: I'm out of ammo! *gets downed and bleeds out*
My Brother: Hurry up and get here! I'm scared!
Random Bandit: "YOU SCARED? YOU SHOULD BE!"
We both burst out laughing.
I join a game in Left 4 Dead 2.
Guy: Hey look its NotAPie, what are you then a cake?
Me: No I'm a Banana
Me: NO I'M A FUCKING BANANA ASSHOLE.
*1 minute of that*
*we are all laughing*
Guy: So what are you?
Me: A banana.
Guy:...You're still a cake.
My last one isn't really a story, there was this one guy on my team called KittenTitz.
Imagine how fucking awkward that was,
"HELP ME KITTENTITZ!"
"SMOKERS GOT KITTENTITZ"
"RUN KITTENTITZ DON'T HELP ME"