For me, a good curve is when you play it for a couple of hours, until you hit a point that's too hard, and you are kind of bored anyway so you stop.

Then, when you come back the next day it feels like the game is way, way easier. You beat the level you tried ten times last night on the first or second go, play for an hour or two and then hit a wall again. Next day, same cycle.

Its important for me to feel like I am getting better, not succeeding because my character is better or I have learned to use some kind of exploit/OP strat.

Games that spring to mind for me: Hotline Miami. Absolute Drift. Superhot. Gran Turismo (Licences and time trials etc). Guitar Hero.

I'd love to know everyone's thoughts in the comments.