I voted gameplay, but truth be told I think both go hand in hand. You can have a game with an incredible story, but if there are poor graphics left and right, and other visual issues like extreme clipping or what not- it ruins the immersion. Kind of like a moving with horrible special effects.
Take Deus Ex: HR for example- the graphics are definitely dated by today's standards yet the game was amazing. The graphics accomplished the atmosphere and environment needed for that game, and didn't take away from the gameplay at all. Quite possibly my second favorite game, right behind the HL series.