Well, I've actually looked into the matter very closely and rather than bore you with a mini-fic, here are my personal findings for Best RWBY Girl:

Ruby Pros: Easily the most adorable. Utterly kind personality and heroic outlook on life. Easiest to get along with. Cons: Youngest and most immature. If you ever made her cry you'd have to deal with a very pissed off Yang, plus Taiyang and Qrow.

Weiss Pros: Graceful and lady-like. Rich family. Those nice long legs + combat skirt = result. Cons: Probably would only accept dates at really high-class establishments. Much nicer than she used to be but would still likely treat you less like a partner and more like a servant. Can be a bit snobby at times.

Blake Pros: Very intelligent. Strong sense of justice. Well-read, always nice to have a girl who can talk about books. Would enjoy being able to gently play with her cat ears. Cons: Cynical and distant personality (though given her past who can blame her). Might bring dead animals into the house as gifts.

Yang Pros: Tied with Pyrrha as most attractive. Tough-as-nails. Easygoing, confident and very funny. Interested in motorbikes and boxing. Cons: Not as bright as the others. Her ideal date is taking you to a sleazy bar... and then starting a brawl.

Pyrrha Pros: Tied with Yang as most attractive. Very friendly and confident. Celebrity status. Cons: A bit too on the serious side. Would definitely make you feel inadequate.

Nora Pros: Very strong. Very funny. Would be really fun to date. Cons: You'd have to watch her all the time to make sure she's not doing anything crazy, weird and/or illegal. People would be freaked out by her all the time too. Would probably buy you a box of chocolates and eat half of them... before you open it.

Feel free to add.