I'm starting this because the "Most time-consuming" thread seemed to have lots of posts about 16 hour smoking, etc. That is not really labor-intensive. Here, I'm talking about time AND work.
I once made a cake from the Alice B. Toklas Cookbook (No, NOT the marijuana brownies) - I haven't seen the recipe for years but it involved, I think, 3 separate cakes that were put together at the end with several different frostings/fillings. It was absolutely delicious but I didn't care. I was so wiped out by doing this all day that I wasn't even hungry.
My partner and I made dolmas for 100 guests when were used to cater about 75 years ago. Our hands were in realllllly bad shape after handling all those salty grape leaves hour after hour.
My nominee for one of the most difficult dishes was from Paula Wolftert (some other chef's recipe posted by her). It was some mint parfait with chocolate placed inside it...you can probably see it online. Amazing!