Exapunks was my first Zach-like and I loved it. It and most other Zachtronics games have a very well-tuned difficulty curve that pushes me out of my comfort zone just the right amount. I think getting stuck for short periods of time makes for a good puzzle game.

I finished Opus Magnum a couple weeks ago and I found it a little frustrating because of the same reasons you brought up. The game doesn't force me to be clever; I can be as simplistic and inefficient as I like. I did go out of my way to design a couple efficient designs, but it didn't feel especially rewarding.

FWIW, my favorite game from them is Last Call BBS. It has several great "mini"games that feel rewarding to just complete.