I think both are important for a great game, but story is still the most important for me. Since I have played quite a lot of games now, gameplay doesn't often seem to be drastically different from anything I've experienced before, so it is unlikely I will be drawn into a game based only on the gameplay aspects. It mostly comes down to the story side of things for me. This includes that characters and the world that the game is set in. If the story is boring, I don't like the characters and the game world is unimaginative, then I will probably leave it alone.
The Mass Effect series is an example of a game that I thought was pretty boring in terms of gameplay, but the story and world was interesting enough that I kept playing just to find out what happened.
I don't know if many of you have played Valkyria Chronicles, but that is an example of a game that has such a bad story and characters, that I could no longer force my way through the game, even though the actual battles were quite fun. Those main characters...good God, they were annoying. I just wanted to watch them burn. Anyway...
I guess there are exceptions, such as Assassin's Creed series and Dead Island. I wanted to know what happened, but those games just got too repetitive and dull.