It’s not necessarily a question of how hard a game is; it’s often more about how forgiving or unforgiving it is. VVVVVV is crazy difficult in places, but it’s also extremely generous.

Slow games: Flower Garden.

There’s very little negative feedback if you don’t play perfectly; flowers wilt if you under- or over-water them, but they never die, and it only takes a single successful watering to get them back to a bright and happy state.