I've seen some people mention that the one highest earner (currently Judy) + the 10 best leveling girls is the most efficient method, but I don't think that's quite the case. I think the two best earners (Judy and Delilah) + the 9 best leveling girls is the most efficient.

As anyone who's leveled enough knows, there are 5 girls in the highest earning tier (Cassandra, Laura, Anika, Delilah, Judy). After ~lv 100, the earlier ones start to get continuously more expensive to level despite all making the same amount at the same levels. So Judy is going to be the one who always gets you to the next stage of earnings, often multiplying your company earnings by four or more when she upgrades. The second one will nearly double that, though, and that's a pretty significant earnings boost. Plus, it gives you another girl to click if Judy isn't available all the time and/or you don't have a bunch of upgrades to make private shows happen all the time. After Delilah, though, it's diminishing returns.

My current team is Amber, Sara, Chloe, Lisa, Alexis, Mrs. Rider, Dominique, Charlotte, Ayumi, Delilah, and Judy. If I were to remove Delilah for the next best leveler, I would be adding Lizzie. I haven't used Lizzie in a while, but that's because I noticed she was very bad for her earning tier, and she would only average ~75 more levels than Delilah anyway. Unless Lizzie significantly improves at higher levels, I don't see this being a good idea.

On the other hand, if I were to want to add Anika for extra earnings, I would be losing Ayumi. Ayumi averages ~100 levels more than Anika, still not a ton, but considering Anika is already noticeably diminishing returns on earnings, it's probably not worth it.

As it stands for me, Judy is the primary earner, Delilah the extra booster, and Chloe and Sara also get surprisingly good earnings for their cost at some higher levels. I've noticed at some points that Chloe both costs less to upgrade and makes more than Anika (which also puts her above Laura and Cassandra) at the same time, but she's never quite better than Delilah as far as I've seen.

Unfortunately, Cassandra and Amanda are my favorite girls and I don't get to use them .