A game system is a set of components that you can use to play a variety of different games. And, as far as I’m concerned, the king Of game systems and the greatest game system is the deck of conventional playing cards. I’d be willing to hear arguments for a challenger to that title (You can definitely make an argument for dominoes) but it would have to be a really, really good argument.
I think a game system needs two things: versatility and at least one killer game. A game system needs at least one game that you’d have still bought the thing even if that was the only game you could play with it.
And a deck of cards has both of those traits in spades (and diamonds and clubs and hearts) You can do so much with just one deck. And it doesn’t just have a killer game or just a bunch of them. It has _families_ of killer games. You have the poker family, the rummy family, the climbing family, the trick-taking family, etc.
Still, it’s fun to look for more modern game systems. A deck of cards is one of the basic building blocks of the hobby, part of its primordial DNA but something more modern can be fun and fascinating.
My personal favorite is Ice House/Treehouse/Looney Pyramids/Pyramid Arcade (I’ve been playing with pyramids with a long time) That said, since you might use dice, cards, tokens and boards in addition to the pyramids, I also view it as kind of a cheat :D Still, the pyramids are a gateway to a wide variety of great games.
I have a ways to go before I’ve explored the Pairs system to have a really good idea how versatile it is or what, if any, killer games it might have. But I am glad to have made it. I think there’s fun in there.