The people. Always the people. Playing a well-designed game with bad people is...sometimes okay, but nowhere near as fun as even a mediocre game with great folks.
That said, rules are still relevant, just not as much as players. A game with well-balanced rules can take what would otherwise be a mediocre GM and some good players and make an amazing game, while a terribly designed game can leech all the fun out of the experience of actually playing the game unless the GM makes a really heroic effort to overcome it.
For example, let's take spotlight time:
A player who's a spotlight hog and insists on everything being focused on them while refusing to let other players shine is awful, and much worse than most rules problems. But a game with well-balanced rules can help to restrain such a player, while a game with poor balance between certain options can enable them and even make players who had no intent to do this do something very like it accidentally. Of course, if it's accidental, it's fixable with a bit of effort (ie: character rebuilding or the like) while something intentional is much less fixable.