I won't dissuade you from trying all of them - I like 'em all :)
SpaceChem will introduce the idea of step-test-debug along with the modular piping (a bit); it has been a favorite for years. TIS-100 will introduce the assembler block programming, and is notable in that it manages to be fun!
Personally, I would start with SpaceChem, as it's fun right away and stays that way for a long time. It always feels like a game, while TIS-100 can feel a bit like work sometimes :P
I'd reccomend InfiniFactory, actually. It introduces you to the step-test-debug cycle, the three legs of optimization (if you're competing for leaderboards), and the general Zachtronics design, much like SpaceChem would, minus the modular piping. However, it's much friendlier to the beginner than SpaceChem (Zach agressively playtested the learning curve, and had more experience than he did when he was making SC), while still leading up to some painfully difficult puzzles that you will probably never solve, as is the Zachtronics way.
Speaking of which, did you know that Zach himself never beat the last level of SpaceChem?
SpaceChem will introduce the idea of step-test-debug along with the modular piping (a bit); it has been a favorite for years. TIS-100 will introduce the assembler block programming, and is notable in that it manages to be fun!
Personally, I would start with SpaceChem, as it's fun right away and stays that way for a long time. It always feels like a game, while TIS-100 can feel a bit like work sometimes :P