Not the timing chain? Nope, its getting the brake booster hose back on the manifold.
Why do I say it is the most painful? One, you'll likely find you need to do it just when you think the timing chain job is all done and your rig doesn't idle right. Two, the go to option you're told is to pull the intake manifold until you can reach it. Three, well the method I outline, no room to show it on camera, will have your arm and hand get a few scrapes and cuts...
I got a pretty decent scratch along my arm getting it in place this way, so be warned, I am not responsible if you get an owie. You can always spend an hour or so pulling the intake instead.
Only a few tools needed:
Flashlight
O2 Socket
A Helper (one that understand that you mean your left not theirs, hard to find)
A Ramp or Jack & Jack Stand