What's the most botted industry?

[Gensokyo]

You have to put quite some work into it, but from a certain point it starts to work on its own. It was extremely satisfying to experience that moment.

That's pretty much my point on why it would be the "hardest", as everything else would generally require much less work, knowledge and generally less combinations of modules or functions.

[Granger]

You might have a point in case you start from scratch. But even then every talented programmer should quickly see the patterns, at least after having implemented storage, smelter, oven, cauldron, loom, farming, handling chicken coops and a basic forager. The rest is just the logical consequence of having a high abstraction level to play with, to see how far you can make it plan ahead, to see how far one can push it.

I don't believe that I was the only one over the years being bored enough to come up with something above and way beyond this script is for farming, this for feeding chicken, this for sorting containers, ... and successfully answered the question of if a bot should need external input with 'no'.

So my stance on the original question of this topic is: most likely everything in the game has been turned into an industry by now, through automation.