That is a tough question; there are so many instances all vying for the worst.
Easily in the Top Ten Worst was replacing the clutch cable on a Starlet without removing the dash- OMFG!
I don't think performing brain surgery would be any tougher or more inticrate.
Replacing the rear bank of spark plugs on Moon Unit while away from home in California without benefit of all the extensions and swivels and whatnot I have at home- OY. I had to remove the intake manifold to do it. Well, I'm not sure it can be done "manifold on" no matter what tools are available. Diagnosing and replacing the water pump on the same car ranks right up there too; everything was backwards and upside down. The t-stat is on the BOTTOM?!? The right front wheel and INNER FENDER must be removed to change the water pump?!? WTF????
Come to think of it, though, heater core replacements are usually even worse. Sometimes you have to lie upside down and turn inside out, and also have backwards flamingo joints in your arms and legs to maneuver down there under the dash or between the engine and firewall, all while putting your back and neck out of joint. Being a giant walrus of a galoot does not help either.
For sheer stupidity, replacing the failed oil pan gasket on the 3.0 V6 '98 Ranger has to take the crown. That was NOT one of Ford's better ideas!
First of all, it had this funky spongy rubber gasket- of the likes I'd never seen before or since- with little metal spacers at each bolt hole to take the torque of the bolts. This brilliant design and material selection SPLIT the entire length of the gasket, so we're not talking about a little oil drip; when that thing let go the oil POURED outta there whenever the engine was running.
Clearly a manufacturing defect, right? I brought it to the local Ford stealership and it sat in their lot about four months while we argued about who gets to fix it. Finally I decided that even if I did get them to do it, they would sabatoge it somehow so I brought it home to deal with it. Long story long, I had to remove the right front suspension and unbolt the engine and raise it up as far as possible without completely removing it in order to drop the oil pan enough to change the gasket. The thing looked like a model car kit all blown apart in my driveway, all for one lousy stupid gasket.
It is worth mentioning that living in the Rust Belt causes just about every "underneath" car repair job to be an adventure of twisted off bolts and other things destroyed by severe corrosion. Thanks for NOTHING, salt trucks!
It has just been -26 deg. F., and that seems to cause cars to demand service even more than usual. I have dropped gas tanks to replace fuel pumps is **** weather like this. I have replaced transaxle differential bearings out in the driveway in this ****, digging a pocket in the snow to work. Yeah, fumbling around with cold metal parts means lots of trips inside to warm up the fingers.
Oh well. I do it for the fun and joy of it.