First, I would like to say, you cannot change your personality. I take that back, you can't change your personality to fit another person's own criteria. What I mean is, (just an example), I like girls who are total freaks; sexually of course. Every girl I have dated was some kind of monster. You on the other hand is the good girl pretending to be bad. I'm going to see through that, maybe not at first but I will. And when time comes to get freaky you give me some school girl act. I'm going to get up, put my jeans on, and call up Jen. So don't pretend, be yourself, it's the best thing you can possible do to not be embarrassed.
Personally, I like girls who are smart, but not smarter than myself. I don't want to be talking about water and she's talking about H2O, we're speaking two different languages. But smart is good, because some girls don't even understand what 1 + 1 is equivalent too. I mean ignorance is absolutely intolerable.
I'm not quiet sure if this counts, but I think most men want a girl who is flexible. This may go back to the "freak" example but, flexibility to me isn't a one-hundred percent must but it would be a turn on.
Last but not least, the definition of pretty is so broad that I have no real way of answering it. I would have to systematically go through every girl who has count my attention to define that. I do not want to get too deep into that also because some people may find it offensive the way I describe the female body. Again, I do not want to get into that, so If I see someone calling me a perv or something I'm going to tag you as ignorant. Since you yourself may have agreed with my ideas if I had expressed them.
However, to answer your question, I do think being "pretty" is what significantly opens up a female/male to the human mind as being a successful candidate. However, some people may claim that what you see online is not what you get in real life, so the definition of pretty leans towards the personal side of the person. Some people actually go crazy when they find out, "She sounded so great on the internet but when we met, she looked like a snake." But for some it may work, I don't know how their minds block out human appearance, but I personality is almost second on my list.