Possibly my favourite puzzlescript game of all time... incredibly challenging but each level can be broken down into mini-challenges that make them feel surmountable and ensure you're always making progress and figuring things out.
I love all the little bits of feedback the game provides in the form of animations and sound effects - rather than stopping you from making an invalid move, the game will show you WHY its invalid. Its subtle, but it really helps with visualising and understanding the mechanics!