you have to either play it or lose to it

This aspect of the definition isn't really useful in Netrunner since it's an asymmetrical game. I suppose a better definition is "What individual cards are must-plays on the runner side that force the corp out of certain options", and vice-versa.

For the runners, I would say Rumor Mill. It has entirely changed the way corps use Jackson Howard, and almost single-handedly eliminated a swath of glacier decks that relied on Ash, Caprice, and even Sandburg, for the short month that it was semi-viable. Even out of faction, corps have to play it safe and just assume the runner could have it, and thus build their decks to be resilient to it.

For corps, it used to be AstroScript Pilot Program, but that has been satisfactorily nerfed. Instead, I would now say Breaking News due to how it can be combo'd with 24/7, Exchange of Info, BOOM!, Closed Accounts, and The All-Seeing I. It enables extreme shenanigans on the corp's part that turn on some very devastating interactions against the runner if it can land. And landing it is not very hard.