Being with the best modeling agency out there doesn't matter if it won't help you. Consider this: Think of basketball...would you want to be with the best team but only be sitting on the bench and not really show your skills- ?
Even if the agency isn't the best, or most busy, or a smaller agency, or a niche agency, might be a better experience for you.
Also you might get more attention.
Don't get hung up on who are WITH in name, what really matters is:
Are they sending you out on castings?
When you call do they know who you are, or are you just another girl calling?
Do they give you honest advice about your photos, ect?
Do they reply and respond to your phone calls?
If the answers are no, then you need to get with a better agency, one that can market you.