Its a hard one.
Obviously the first thing you notice about someone in their looks, but looks fade, and after a few months, personality definately matters - I've went out with some really nice looking guys, only to find out after a few weeks into the relationship, they're as thick as shit and cant engage in a decent conversation with me and when that happens, you suddenly dont find this person attractve anymore because all you see when you look at them is stupidity.

I hate good looking people that love themselves and walk about with an air of conceit, I mean, what a turnoff. Get over yourself, it doesnt matter how gorgeous you are, if you are conceited, you arent going to get very far.

Anyway, to cut a long story short, if you're looking for a long term relationship, personality is the most important aspect.
If you're looking for a one night stand then I guess it would have to be looks.