Both, one without the other sucks. It may have good gameplay but it could have a story that doesnt make you want to see it through to the end. Or it could have a great story but gameplay so boring and repetitive that you cant be bothered to continue.
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You can't pick both, not many games can balance both.
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Neither one of them has to be top notch but they are both equally important. But if I had to pick one I would lean more towards the mechanics of the game. Make a game that is easy to play hard to master type of thing.