URs by Center Skill

Simply take your desired team and count how many members of each type it is comprised of.
Types are group, subunit, and year group: you can see those Types by clicking on a card at the top if you don't know them by heart (highlighted in the screenshot below).

To give a clear example, here's my Smile team.
Here, I have:
9 µ's members
4 3rd year members (Eli and Nico)
4 1st year members (Rin and Maki)
1 2nd year member (Honoka)
6 BiBi members (Eli, Nico, Maki)
2 lily white members (Rin)
1 Printemps member (Honoka)

To know which center skill would be the most optimal, you need to look at which Type (excluding main group) has 5 or more members, as that is the optimal minimum. Here, I have 6 BiBi members which is above 5 and is my highest amount, which means that for my Smile team, a BiBi boost would be the best.

However, another key point is that with 4.0 and the introduction of School Idol Skills, on-attribute center skills (Smile Princess, Pure Angel, Cool Empress) are much, much more efficient than off-attribute center skills. Hence, for each attribute team, it is better to choose an on-attribute center skill along with the secondary boost that fits your team the best. But, if you use many off-color cards in one of your teams, you might want to calculate the 12% and see fi it beats a 9%.

We've said that for my Smile team, a BiBi boost would be the best, which means I need a Smile Princess BiBi boost: that would be Kunoichi Nico. Hence, when I play Smile songs and want to focus on getting a high score, I will choose Kunoichi Nico as my guest center.

But as you can see, I do not have a Kunoichi Nico in my team, but I have two other URs. In order to find which one is the best as center of my Smile team, simply see the Type counts above: I have a China Dress Nico which is a Smile Princess with a 3rd year boost, and November Maki which is a Smile Princess with a µ's boost.
I have 4 3rd year members, which would be a 4*6=24% boost, whereas the µ's boost would be 9*3=27% boost, hence where the µ's bonus from November Maki is better.
(this simple math is also why at least 5 cards of a Type are required to be more optimal than a group boost: 5*6=30% > 27%)

Use this page to find which cards fit your teams in the most optimal way!