As the title says, what matters most to you in a game? Is it a deep interesting storyline? Is it fantastic gameplay? Is it the quality of the graphics? Or do you like games for their difficulty?
For me, I'd have to say gameplay. The game could have a boring storyline and 2-D graphics (actually, I'd encourage it. I don't really care much for games that focus on graphics.), but if the game is fun, I'm hooked.