Servicing cup and cone bearings.
It's a pain to get that extra fine balance balace between free running and no side-to-side sway.
My second hate is replacing gear cables - always a pain to get the gears perfect.
This sums up the problem with doing almost anything on a bike. Virtually every fixing doubles up as an adjustment, so you can't dismantle anything without having to faff around readjusting it when you put it back.
FWIW the way I deal with cones is first screw them all the way down finger tight. Now put the spanner on and back them off using the spokes as a protractor to note the angle you've turned them through (10 deg per spoke). Once you've found the right setting, note down what the angle is and then you can reset them again and again using the same angle without the trial and error.
Bear in mind that the skewer also affects the cone adjustment by compressing the axle, so you need to tighten the skewer consistently too using a similar method.