Ask HN: What are some of the best take-home coding tasks you've gotten?

As an applicant, I've personally had a mixed experience with take-homes. We're designing the technical interviews right now, and I thought I'd ask for some stories on exciting take-home tasks. We're looking for something that resembles the actual job, so we'll allow any tool (including AI and debuggers).

Also curious to hear about any bad ones you've done.