Gameplay to me is what makes up the experience of playing a game. A lot goes into that, and just calling it 'gameplay' is to be a simplification that in this context makes it impossible for me to 'pick' one over the other. Music, visuals, design, story, all of this goes under the "umbrella" that is gameplay.
So I can say that I think 'gameplay' is the most important aspect of what makes a video game good. But you can't have gameplay without visuals, design, music and story. You can combine those elements and use them differently for each project, sure, but to try and make one more important than the other, that to me shows a lack of understanding of the topic at hand. Gameplay can't exist without its ingredients, and if you want to fairly judge a game you have to take all of its contents into account.
So when someone tells me that 'the gameplay is good', that doesn't really tell me anything. Sure, it tells me they've likely enjoyed the game on some level, but it says nothing about the how and why, and about what makes it great. Gameplay isn't "more" important than graphics, because the graphics is a part of the gameplay. To see a good example of what I'm talking about here look at a game like Journey. That game has a lot going for it, but if you boil the moment to moment 'gameplay' down, it becomes nothing more than a 'walking simulator'. It is the visuals, and more importantly the music, that makes that game work. If you strip it of those two elements, the 'gameplay' suddenly loses its edge, and there's really nothing left to enjoy about it.
What I'm trying to say here, is that saying that 'the gameplay is good' is effectively the same as saying 'the game is good'. But it skips the interesting part, which is why.