Chess.
For most games, it's not difficult to make AI that can absolutely destroy humans. But it turns out to be very difficult to make AI that feels like a fun and engaging challenge to a human. Hardest of all is making AI that realistically plays like a human does.
Currently a Galaxy S9+. Battery life isn't the best anymore, and I'm running low on internal storage, but it's still serving me well. I'm not even sure what I'd upgrade to, I really need the SD slot that almost nothing new has anymore.
Best was my old Motorola Droid 2, I miss slide-out keyboards so damn badly.