IDK about MOST challenging, but this one sticks out in my mind as among the most aggravating.
TLDR: '00 Olds Silhouette (Chevy Venture / Pont Montana) power steering lines. Rack end fittings had no access except from the top, putting me shoulder deep. Very stubborn fittings and I swear they were 17.5 MM - nothing fit well. And there was pretty much no room to get in there with anything but a wrench.