Super Mario Bros, Pac-Man and Tetris seem like the three most obvious answers to me.

On the PC side, there really isn't much. Even at their peak they never seemed to permeate the culture as much as arcade and early console games. I'd say the closest they got were Doom, Myst and The Sims, but I wouldn't put those with Mario, Pac-Man and Tetris.

Anyone who plays Farmville is dead to me.

anybody who doesn't say super mario bros or tetris is wrong

To add to my earlier post, Space Invaders and Donkey Kong are also reasonable answers.

Assassin's Creed. Best series ever made.

Wrong. [Insert your favorite series here] is way better.

:-) Agree.

the 70s rool

edit
What about Ultima. Its like Star Wars in that many have never played yet many would swear it is biblical

Heroes of Might and Magic, Doom, Age of Empires, and Warcraft.
Begrudgingly, Starcraft I guess.

Doom, Starcraft and Warcraft, maybe. HoMM and AoE are nowhere even near the cultural ubiquity of Star Wars.
Same for Ultima. I mean, it was big for a classic PC RPG, but again, we're not talking about the kind of cultural relevance that has people making casual references to it decades later. You don't just ASSUME that your co-workers know what Ultima is, you know? But you can pretty fairly assume they'll know who Mario or Pac-Man are.

You could pretty much pick a thousand games to fit the bill, but I'm going to choose one. The one I'm going to go with, is pretty much recognized as the trailblazer for at least one genre, if not more. Doom.... yeah, I know it wasn't the first FPS, but it just took gaming to whole new level when it came out. From the graphics to the audio, just about everything in that game/franchise influenced what came after it.

There is this weird trait that many (not all) young people tend to have that they don't consider anything relevant that isn't born of their own era. It's natural really. Teenagers strive through culture to find their own voices and identities, so Jethro Tull probably is just grandpa's music compared to (insert whomever is hot this week).

But I think people with strong interests in music, as they age, start to widen their tastes and discover that a lot of older music begins to be as important to them as their own generation's sound... especially when they start to hear older music's influence on newer artists.

The converse is true for some (certainly not all) older listeners who get into a state of arrested development where is seems that whatever they loved in college was the last thing they ever thought was good. I hear too many people my age and older bemoan how "they don't make good music anymore" which is total BS.

But games age poorly for many people. You guys are are pretty serious gaming enthusiasts so playing a twenty year old game that lacks the cutting edge graphics and features of a modern title is just as interesting to you as whatever is new... but games also age poorly in that they get harder to run as technology marches on.

I mean, sure, you and I can run Dungeon Keeper in DosBox, but most people like putting in a disc in and having it work. They aren't dumb, they just aren't as interested in the challenge of making it work to discover some lost gem when they could just start up Red Dead Redemption and play it right now.

But more people become gamers ever day, and I've heard it said by more than a few industry experts that the day is soon coming when "everyone" will be a gamer because everyone will be someone born into it. Classic gaming might be on the rise... and if it is, I think every gamer should experience:

Interstate 76
Dungeon Keeper
Super Mario Bros 1 and 3
Syndicate
Civilization 2
Interstate 76
Carmageddon (1 and 2)
Dune
Baldur's Gate (though I'm not personally a big fan of those games)
and especially, Interstate 76

Alan Wake. It's what the cool kids play.