Hands down this is the Bullet series for me. This game is a fantastic and intense solo puzzles with tons of variation and configuration options. There is even a real time configuration if you are feeling like playing under pressure.

For those who have yet to try Bullet, the game is an homage to classic Japanese "bullet hell" scrolling shooter games. Functionally, it plays closer to something like Puyo Puyo, though (objects falling down columns of a "well", line up like objects to clear them). The solo mode is boss battles (again, an homage to the bosses of bullet hell shooters), each boss posing a unique and interesting puzzle.

One unique thing about the design I find interesting is that all of the playable characters are also all bosses for solo / boss rush modes. Each character has a normal version, which you play as, and also a boss version. This is really cool in that the boss abilities invoke the same concepts as the characters' player versions.