For me - my least favorite is certainly plant processing & gem encrusting. Both of those are hard to control without investing significant time into stockpile management and burrowing. On smaller fortresses it's easy, but on huge fortresses like mine (350+ pop) it is a pain to search through the whole list of dwarves to find my plant processer.
Favorite is probably steel-making. Straightforward, you can control the entire process, etc., instead of "DAMMIT DORF YOU WERE SUPPOSED TO PROCESS THE DIMPLE CUPS BUT YOU INSTEAD PROCESSED MY WHOLE FORT'S SUPPLY OF SWEET PODS AND CAVE WHEAT THAT WAS USED FOR MY DAMN BREWING" sort of thing.
The most important aspect of any dwarven industry is stockpile management.
It only takes significant time to learn how to manage stockpiles the first few times, like any learned skill.
After you get the hang of it, it becomes second nature and takes very little effort.
The amount of time it takes to wait for dwarves to do things with inefficient stockpile management by far outweighs the time to manage stockpiles properly.
There is really no need to have 350+ dwarves when you can easily break game economy with just 30 properly managed dwarven workers.
That being said, there is very little difference in managing stockpiles for 30 dwarves or 300, the only difference is in scale.
In fact, it is even more important to properly manage a large number of dwarves than a small number of them.