See now, this is... a trap!
We can't say looks anymore even when it's generally the most obvious thing, because that's objectivist, isn't it? Mm.
... I'm going to vote laugh. Generally there is no one thing that immediately... no, that's not right.
Are we talking initially, or what we're looking for in a long-term partner? Because the latter is obviously personality, and the former is obviously looks (for the most part anyway; if you meet someone over the 'net without knowing what they look like then it'll be personality again, wouldn't it? But how often does anyone pursue a relationship without knowing what the person in question looks like? Sure, it shouldn't matter if everything else is perfect, but it does.). Could there be anything worse than living with someone with whom you couldn't gel with whatsoever?