Town of zzt by Tim Sweeney (1991)
Town of zzt is one of the original worlds shipped with zzt. And it's a good one. Many mechanics of zzt are explored in it, from slide-puzzles to bombs, hell there's even a basic musical puzzle. Overall I remember it as being genuinely fun, if not a bit too hard in the lake with bullets section (left of spawn). Zzt controls, at least in dosbox, are a bit clunky; so they're not an ideal combination with a dodging section requiring precision.
Verdict: recommended. Essential to understand what zzt is.
I also played city of zzt, which however I remember barely. It was ok, but you can probably skip it, unless you're interested in checking out all the original worlds.