which are the best girls to level up?

[Siclop]

which are the best girls to level up?

[Attirm]

Someone has produced a wonderful spreadsheet resource that goes over all the stats of the girls: https://docs.google.com/spreadsheets/d/1kVZxcZZMa82lS4k-IpxTTTELAeaipjR_v1twlqW5vbI/edit#gid=1661376645. The "Max Base SUM" column is the one that shows the combat stats of a girl. The higher that number, the more powerful that girl is in combat. The other consideration in a girl's power is what qualities she has, and how likely she is to get blessed. This is what the "chance to be in the top 7" is trying to represent, however this column is only useful if you have most of the girls in the game in your harem. The more girls you're missing (particularly mythics and 5-star legendaries), the less useful it is to you. The most important thing for a girl's strength is the number of stars she has. Notice that 5-star commons are as strong as 3-star legendary girls.

Another important thing to note is that higher rarity girls require more books to level up and significantly more gems to awaken. This means low-rarity girls with a high number of stars can be very good deals, costing very little to awaken. At the same time, high rarity girls with a low number of stars are terrible deals. A 3-star Legendary girl is only about as strong as a 5-star Common girl, but costs four times as much to awaken! This means it's a very bad idea to awaken 3-star girls. 

You also have to consider your elemental balance. Different girls have different elemental types, and will require a different color gem to awaken. You have very little control over what color of gems you get, so you want to balance the girls you are focusing on to match the gems you're earning. Try to figure out how quickly you're earning certain kinds of gems, and focus a little more on the girls that match the type you're obtaining more often. I keep a spreadsheet to track who I am levelling up and how many gems of each type I will need so I can see problems before they happen and don't start levelling up a girl if I can't afford to.

The pyramid is the final consideration. In order to awaken any girl to level 300, you must have at least 20 girls at level 250 to awaken them to level 300. Then you need 30 girls at level 300 to awaken them to level 350. This means you need to awaken a very large number of girls to unlock each new level threshold. This creates an interesting tradeoff: mythic girls and 5-star legendary girls are the strongest girls, but are very expensive to awaken so if you focus on them it will take you much longer to reach higher levels. Lower rarity girls are weaker, but cost less so you can awaken them more quickly. This can help your strongest girls reach higher levels sooner. I think the right approach is to mix both approaches, use some strong high-rarity girls and mix in some cheap low-rarity girls.

TL;DR: Mythics girls, 5 star legendary girls, and 5-star epic girls are your best girls for battle power but they're expensive to level up. 5-star rare girls and 5-star common girls are less expensive but also less powerful. You can use 3-star or 1-star common girls as cheap ways to level up lots of girls and meet the requirements to awaken to higher levels, but they are very weak and mostly useless in battle.