Lingo. It tickles my brain in wonderful ways. I’m currently working through the custom level Liduongo, sequel to an earlier map named Duolingo, and I continue to be surprised, delighted, and utterly perplexed.
It’s a rules-based puzzler that doesn’t tell you the rules buried in a confusing labyrinth. The only downside is that it requires a strong grasp of English, limiting its audience.