Gameplay... it's a game. People still enjoy Tetris after all and Space Invaders and all manner of clever old games that don't look necessarily fantastic but are still addictive and fun because of the mechanics and challenge.
Graphics are like... the cover of a book. It should look nice, and reflect the quality of the product inside, but if you open a book up and it's crap, you're not going back to it no matter how nice the presentation is.
Definitely Game Play. This is why things such as Minecraft are successful.
Recently I have experience good graphics and terrible game play in the form of Doctor Who: Worlds in Time. It looks great but it is absolutely awful to play so I played it once and never went back. Just goes to show you have one chance to capture someone in a game and if you fail then that's it.
I would have to say both.Game play is important for any type of game ,but there's nothing wrong with a bit off eye candy as well.I have played some fantastic indie games which had better gameplay than most aaa titles , but the replay value is not that high when the gfx suck.When i play a game for the first time i usually play to complete it.When i go for a second time then i want to admire the gfx and artwork that went into that particular game.
Casual games can get away with fugly gfx , but hardcore games need hardcore gfx to accompany it.