The biggest, and probably most popular, of the current mobile Idolm@ster games. It has some of my favorite girls and music, but I'm not sure if I'd definitively say it's the best of the games. Top 2. (Shiny Colors being the other, but it isn't a rhythm game.)

Pros

• The girls. There are 190 girls, so you will inevitably find at least a few that you really like.
• The music. There are currently almost 400 songs and, in my opinion, Cinderella Girls has the best overall music quality of any of the iM@S franchises. There is tons of variation thanks to the large number of idols and a lot of very talented voice actresses doing the singing.
• The stories. Every girl has an introduction arc and short solo stories that come with the cards, but the real pleasure is the large number of stories with various girls interacting with each other. There's no overarching narrative so it's basically CGDCT and typical Japanese feel good stuff.
• The art. The artwork for the various cards is beautiful, usually. Some cards are meant to be goofy, but they're still high quality art.
• If you don't feel like playing but want to blow through stamina it lets you use auto tickets that replenish every day. This way you can farm event points and such without dedicating tons of time to the game.

Neutral

• The gameplay. Being a rhythm game, it's skilled based. You can clear even the hardest of songs with almost any team if you're good enough. On the other hand, if you're really mid like me, you might never clear songs above a certain difficulty and therefore never be competitive in rankings. This doesn't stop you from getting anything important, but it can be demoralizing.

Cons

• There are 190 girls, so you will inevitably find most of them to be meh and some annoying. There are also still some unvoiced girls unfortunately.
• It's a gacha game. The drop rates are okay and it gives out a decent amount of free currency, but gacha is still evil.
• The rhythm aspect is less forgiving than games like Million Live and it does glitch and occasionally drop notes.The dropped notes are by no means constant, but it can be annoying when it ruins a perfect combo.