I voted graphics.
But what I really mean is the engine. If there are cool physics, good graphics, good sounds and intelligent AI, it makes the game fun to play. But the best is if the physics somehow contribute to gameplay, e.g. Red Faction: Guerilla.
If a game has ultra petty engine, but awesome gameplay, the game cannot reach more than a "meh" for me.
On the other hand, if the game had petty gameplay, but awesome engine, I would probably like the game much.
RF: Guerilla is case in point. The gameplay isn't particularly genius, you shoot people, get money, buy bigger guns, and shoot more people. However, the fact that it is sandbox is a plus for me. But to the point, with the GeoMod 2.0 technology, the game gets immensely interesting and fun to play. Without the GeoMod 2.0 technology, I probably wouldn't like the game that much. Volition physics engine isn't anything to brag about, but the GeoMod 2.0 feature rox0idz.
Addendum; I see comments that state that games today are pretty. Well they aren't. Few games look pretty compared to the advancement in technology. Our technology is able to help us surpass the Uncanny Valley, yet we still aren't even close.