Sunflowers

Six difficulty tiers run from beginner up to insane, and the supply of puzzles is unlimited and free. The game underneath is built on number clues: you read the numbers and work out where the sunflowers belong in a virtual garden. Every grid comes out of a logic engine that guarantees a solution reachable without guessing, so a stuck board is a reasoning problem and not a coin flip. There is a daily puzzle as well, and progress is tracked as you go.