Any simple mobile games worth playing?

I'm a big fan of Destiny Child. Complex when it comes to learning it, but simple when you have it down. Korean developer, if I recall correctly. There's like 10 different currencies that each do different things, and that's not including items that I guess you could count as currency. Once you know what they are, know how to use them and know how to get each one, it gets pretty easy. It's gacha, but it's easily the most fair gacha I've ever played when it comes to premium content. Haven't spent a dime and I'm not falling behind because of that.

BATTLE CATS!!!!!!!! It's a tower-defense style game, japanese, and if you play it right, you don't ever have to spend a single cent to get a shoutloud of the premium content because they give away soooo freaking much for free. I have a lot of advice about it, because I've been playing for several years, but my biggest two pieces are this: ONLY spend catfood to unlock the Special Units or on Rare Tickets for the 11-cat draws when it says "100% Chance Uber Super Rare!"