Come ON people. The most obvious one EVER has to be:
"Press X to talk."
Someone should have said that.
Other than that it should be a pretty general one, not everyone plays Bioschock, or WoW, or whatever game you pick it from. It has to be something found in all, or at least enough games, that it's recognizable anyway. It's much easier for Trekkies, or Star wars fans since it's a single work, a single fiction, and everyone will have seen the same thing. Use the force, or live long and prosper works because they're in everything.
When it comes to gaming it's all just one game. Sure, everyone who's played Half-life knows Black Mesa, and probably a lot more people too, but it's still excluding people who only play other types of games.
Edit: Otherwise, "I maxed out charisma, what have you got for me?" is nerdy enough.