I think that gameplay definitely comes first. A game could have a great story and great graphics, but if I don't have much fun playing it, I'll play it once and never again. Good gameplay is what makes a game fun the first time round AND second. I think that this and difficulty are similar; a game could have good gameplay but if it's too easy then it'll just get boring. If it's too hard, it'll be too frustrating.
I like a good storyline but it's not essential. In fact, I think some of the best games have mediocre storylines. Take Mario, for instance. The game's based around saving a lame-ass princess from a monster. There isn't much of a story, and what there is is, but it's a great game. Banjo Kazooie, another one of my favourites - Saving the main character's little sister from a wicked witch. Crappy storyline, great game. I personally think that FFX's storyline is mediocre (compared to other FF's) but the gameplay has me coming back for more.
I don't care much about graphics, although good graphics are a nice accessory.