I would say that both are important to me in their different respects.
Graphics: Of course I can't really say this is completely important to me since I play a lot of rogue games such as; 'Caves of Qud' and 'Dwarf Fortress'. However graphics are still pretty awesome even if they are only a side feature, they do make a game better. I'd love to say play Dwarf Fortress with really good graphics. Why? Because they help me immerse in the game world, I can play games with 'bad' graphics for a time but sometimes I find it hard to immerse myself because I don't find the world believable aesthetically. So in my opinion, while graphics aren't the most needed thing in a game they nearly always improve the game by making it more aesthetically believable.
Gameplay: Well, of course you need this. Gameplay is the core, the heart of any game and without it (on a decent scale), the game will mostly flop. I prefer my games to have deep gameplay elements such as Dwarf Fortress which tracks individual body parts and damage along with completely randomly generated worlds and your characters in the world can have impacts on future playthroughs. Example: I create an adventurer who is a dwarf and preforms a lot of amazing feats, when I build a fortress later on in the world's history, the dwarven civilization might worship him. To me, this is an example of excellent gameplay.
So, in my honest opinion, both are pretty much equal with gameplay just being slightly more important.