The last 3 girls in texts are the new ones, the 5 before that are the "best" girls, with the last of the 5, Judy, Being the best.

FYI, there's also a decoration that reduces base cost, and since level up cost is a factor of base cost, it's pretty sweet. Reduces based cost by 2% per upgrade, and upgrades significantly cheaper than the level reduction one.

lvl 20 (-40% base cost) with 4.37k upgrade cost vs.
lvl 9 (-9% level cost reduction) with 3.11k)

Not really all that sure which is better. Depends on precisely how the level up costs is calculated. My gut is the base cost reduction is significantly better.