I guess something that can still hold it's value after so many years. That's why it's impossible to say what's an "instant classic" because you really don't know that for sure until it's beaten by age. Sure the Matrix seemed awesome at the time, but come on, it ain't as good if you rewatch it today.
From a video game's standpoint, I believe a game needs to be fun even years after it's release. Mario Bros is still fun, challenging, and can be quite addicting. Silent Hill is still very scary, and looks surprisingly good even today. The material it works with is heavy, yet it dealt with it in an appropriate way. Also, speaking of Silent Hill, I do absolutely love how Silent Hill never dates itself. Silent Hill 1-3 never really make it all that clear when it takes place. It could be from any time, and that's another thing that needs to be taken into consideration when talking about story-heavy mediums, if you want to be TIMELESS, then try to make your story relatable, but also try not to make pop culture references. Unless your a comedy, but even that can seem dated later on.
Ocarina of Time is something that is considered as a Masterpiece and a Classic, and I agree. But why exactly? Other than the fact that it was the equivalent of Skyrim at the time, Ocarina of Time is just, well, an epic tale. You're a child who goes through time, fighting evil from both your period of time, and from the future. It's incredible really. When people say "epic" they usually think of the overused term, something that's like, BOOM! HUGE EXPLOSIONS! GIANT MONSTERS!! OH MY GOD, EVERYTHING IS BLOWING UP!!! LOOK AT MY BIG SWORD! IT'S SO EPIC! In reality an epic story is one of heavy and legendary importance. One that effects the world forever, one that will be remembered forever. And I think this is why OoT is considered one of the greatest games of all time. You're a child who travels the world, and in the eyes of a kid, the world is a big and scary place...but Link's brave enough to go on and face the danger. Like a man.
I hate it when stories are given that title. "THIS IS THE GREATEST ________ OF ALL TIME!!" because now we've just subjected that piece of art to criticism, and a LOT of it from a LOT of stupid people. This isn't bad, criticism is welcome, but when something is labeled as the GREATEST OF ALL TIME then we get super stupid and pretentious critics who say "IT'S SO OVERRATED" or "THIS SUCKS SO MUCH, IT'S NOT EVEN GOOD". Citizen Kane was considered to be overrated by many critics, but again, I do believe they were just wanting to deviat from the norm. Honestly the story for Citizen Kane isn't all that great, but what makes that movie a classic is it's AMAZING artistic direction. Oh my God, that movie does some awesome shit with a camera, I mean, holy smokes. You don't even see most modern movies do what Citizen Kane does. This is what makes Citizen Kane a classic. It's sheer beauty is one to be studied for years to come.
There's a lot of misconception that Classics HAVE to be about a normal day guy, a "relatable" character. You can have relatable characters be super human, ya know. Human nature is something we can all recognize, even if your main character is a blob or an animal, if they display feelings like a human, struggle like humans, and just overall BE human, I think we can get away with any main character being not-human. Take for instance Frank Miller's Batman: The Dark Knight Returns. Everything about Batman isn't normal when you delve deep into his psyche. He's a paranoid, violent, and psychotic man that just so happens to still retain his sense of morality. The novel focuses heavily on the struggles of Batman, and how he deals with his old age and the new street punks that infest Gotham. Not all classics have to be about feeling good, some are about just feeling. What Batman deals with can correlate to anyone anywhere. Everyone deals with age at one point in their lives, and everyone deals with the "new" and acceptance of it.
In the end, Classics are something that can be found several generations later and still be worth the read, the watch, or the play. It's about being human, and about being timeless. What you think is amazing now will degenerate later on. Always focus on the element that humans never forget, and that is emotion. Fashion, media, celebrities, they all come and go. Many are forgotten, some are immortal, but never you mind them, always remember to stick with what you feel.