ZZT. I think a modern ZZT would make for a great educational tool to teach coding. A lot of existing, popular tools exist at either end of the spectrum of difficulty: too easy or too hard. ZZT was simple to design, fun to draw in, and the coding wasn't too complex.

Megazeux, a spiritual sequel by another lot of people, I always found far too complicated. It's not that different to ZZT, it just manages to be fiddly without a good enough pay-off in my view.