For a serious game I’d recommend UnCiv, imho the best adaptation of Civ to the mobile form, even counting official games. Very frequently updated.

For casual games, I like:

Lona: like Snake from old feature phones, but with circular motions. You’ll understand it when you see it. Familiar and fresh gameplay at the same time. Difficult to master and keep beating your highest scores. Very addictive.

Vector Pinball: as it says on the tin, just an honest to dog, simple pinball. Many tables to choose from.

I’m also addicted to Poland can into space, which is a stupid game with bad gameplay, but fun nevertheless. :)