If you mean in a positive way because she is attractive, then "a looker" or "a head-turner" might be what you are looking for. If you mean her work ability/qualities, then you could say "outstanding" or "head and shoulders above the rest". If you mean in a very obvious (maybe negative) way, then "she sticks out like a sore thumb". Also, "conspicuous" would be a fairly neutral term. I'm sure there are plenty more expressions (which also apply to males).

The first word that comes to mind is "striking." It can be used to describe men or women and also objects ("I like this artist, his work is very striking.") Also: attention-grabbing, distinctive, memorable, conspicuous.

It depends on WHY she stands out. It is a judgement call. For me, an ATTRACTIVE or PRETTY girl stands out in a crowd. ;) But also, a LOUD or BOISTEROUS girl would stand out. TALL or ENERGETIC of ACTIVE or SMART, etc. It depends on what you notice about people, what makes you look at them.

independent (outsider)

Social and able to make friends limitless