It's a trickier question that it looks, and there's little doubt that in an ideal situation we'd have both. I'm more partial to a good story myself, but many of the best games make an attempt to involve the gameplay in said story. It's what separates the medium from the less interactive forms (film and books). For those who have been to live theater performances, you may have noticed that many of the best shows seek to involve the audience (usually by talking to them briefly, or occasionally having the actors being among the seats); gaming just takes this further by getting the player directly involved in the events. Films & books never change when watched/read again, but games & theater are very seldom the same thing twice.
Now then, bad gameplay will kill a game where a bad story may not; but while it's definitely more essential to game design, it doesn't necessarily have to be what the player prefers. Many games have gotten by with merely passable gameplay while a great storyline provides the motive needed to complete a game, an aspect which is very prevalent in the RPG genre (which is generally regarded as having the best storylines in gaming, collectively speaking). Back to games not being the same while replaying them, strong gameplay will likely play a bigger role here than a strong story. Bland & dull gameplay easily falls into repetitive routines (which can make a single playthrough too painful to complete, let alone multiple ones), while high-quality gameplay typically offers a lot of variety and you're able to approach things from different angles; it's that reason why at least passable gameplay is necessary for all games, even story-driven ones.
In the end, I think comes down to a matter of perspective.
Should the story complement the gameplay?
Or should the gameplay complement the story?
I definitely prefer the latter situation, because fun-factor alone doesn't keep me playing games. I'd very much prefer to have a great story for a game, and the experience is made better by the gameplay allowing me to be more directly involved in it. Fun gameplay is great, but it's just not something I can enjoy for it's own sake... though it doesn't take too much of a story to keep me interested enough to finish a game.