Beekeeping, every animal skill besides training (trapping gets a pass if I want to make a zoo or aquarium, but otherwise useless), fish dissection, glazing (I don't bother with any clay except fire clay, since I can make rock pots and jugs without a fuel source just as easily, I don't need an extra step), and pump operators (I only enable this labor when I need a pump temporarily used, so a dedicated pumper is pointless, or to give my soldiers some exercise).
Strand Extraction only has one purpose, but a novice or dabbling extracting is so damned SLOW. I want those strands extracted YESTERDAY, dammit! However, only really need one good one, dabblers or novices can support the one good one easily enough.
Since it seems a better Furnace Operator smelts faster, I actually love legendary ones. Three legendary smelters can turn a humble pair of magma furnaces into a full-tilt foundry, and keep a bustling metalworking industry thriving.
The same goes for other sorta-useless skills: milker, spinner, shearer, and so on. I want those jobs done fast, so one highly skilled one is a boon in a larger fortress (I like to play with over 100 dwarves), as a legendary shearer can keep massive flock under control. All that yarn keeps me in bags so my rock nut pressing and sand collecting never stop.
Lye maker and soap maker are often the same dwarf; only if I get one REALLY good lye maker and one REALLY good soaper will I bother with two (again, purely out of a desire to crank out soap quickly).
I actually use potash maker sometimes (since I like clear glass for display cases and arenas and skylights), but it's not something I need a dedicated dwarf for. I usually take one, repurpose him as a hauler, and leave it enabled for when the time comes.
I actually have two hauler classes: "Hauler" and "Laborer". Haulers just have hauling, cleaning, and engraving enabled (for mass smoothing; when I want actual engravings, I disable the labor from haulers and let my stoneworkers do the rest), and are always the cannon fodder for my city guard and soldiers. Laborers are haulers, but with one or two other semi-useless labors highly skilled and therefore enabled (like Potash Maker or Milker). The distinction is there purely so I don't accidentally draft my Legendary Lye-Maker/Soaper, and get him killed and get stuck taking forever to make soap again. However, I'll be damned if I let a Legendary Potash Maker loaf around the dining hall, getting high social skills and elected mayor. No no, grab some refuse and get hauling, buddy! I'll call you when there's potash to bake.
EDIT TO AVOID DOUBLE POST: Also, had to point this out:
What, no mention of Alchemy?
Uhh...
Potash making and the two dissections. I've used every skill at some point but those and alchemy (and alchemy in some mods).