Girls in Monsey tend to go largely to BY Monsey, Ramapo, Breuer’s, and Ateres in Monsey and MHS and Bruriah outside Monsey (I obviously know people who went elsewhere but not in particularly big groups).
Which elementary school your daughter goes to makes a difference. Some will encourage some schools over others and some will provide an edge to get into some schools over others (it’s harder to get into Ramapo from Ateres than from YSV, for example, or it was when people I know tried a few years ago).
If you want intellectually stimulating, you may want MHS. (As I basically already essentially outed myself on here years ago, I may as well continue and say that I went to MHS from Monsey, if you have any questions.) The hashkafa might not be what you’re looking for (while it is right-wing it is not explicitly BY and there are some non-BY girls, to be sure), and the types of girls there really depend on the year, it easily has better academics than any Monsey school. I really enjoyed it, though I’m shocked at how much elements have changed since I was there not particularly long ago.
Bruriah also is out of town, has great academics (though in a different way than MHS) and is probably the most “Modern-with-a-capital-M” school on the list. I have several friends who went there and loved it.
The next best academics is probably Breuer’s. They seem to be doing a really good job in that regard, and are known for it. It is definitely a Bais Yaakov but more chilled girls feel comfortable there. I know people there and they definitely like it. The class sizes are getting smaller and they’re getting more selective, apparently trying to become more BY (though I haven’t personally seen the ramifications).
Ateres is much more chilled. The academics are probably similar to Breuer’s’, with the addition of AP and more college-level classes (including a Touro partnership). The population of girls will depend on the year. It has BY hashkafos to whatever extent but the population is largely not BY. This is a fact, not at all a condemnation- you just have to know if it’s for you. It’s definitely very warm, very relaxed, etc. The dress code aspect is good (they copied it off of MHS, which has the same thing). I have a lot of experience with the school and it’s great for many but not for all (which is the same for any school, really).
I know very little about BY Monsey currently (everything I’ve been saying about the other schools comes from personal or close second-hand experience, which I don’t have with BY).
Same with Ramapo, except that I’ve heard that they’re trying to become much more yeshivish. But like I said, I have little to no first-hand experience.
Good luck! If you have any questions, feel free to follow up!