Since going out on my own I have practically lived in crawl spaces, be it above the ceiling or below the floor. Just finished an attic job a few minutes ago actually though there's nothing to report on that one. (pause to scratch)

All of the usual sweat, blood, tears, and profanity ( not always in that order, but usually) apply of course.

The worst two that come to mind are under the houses.

First runner up had me learning much about the territorial habits of Black Widows and breathing a lot of bug spray.
Doing a complete re-wire of and small old house that was completely loaded with colonies of Black Widows underneath. I would nuke a colony (so as to approach, drill up and shove a wire up) only to find a competing colony in place at my next up-hole. This is right after speaking with an HVAC installer who told me about how he was bitten by a Black Widow in the face and how it took a year or so to regain natural facial expressions.

The winner is----

I was hired to re run some RG6 in a house that was being spruced up for a flip. I took a quick look in the access hole and said "no problem".
After planning my strategy for minimal in and out trips to get to and from where I needed to go, I entered the crawl space. The height of the space was such that outright crawling wasn't possible. Only the boot camp, crawl under the wire while they shoot live rounds over your head technique was possible, with the occasional drop to your belly and go under the plumbing moves thrown in of course.

This would have been routine if it were not for the ground. It was like a dried out lake bed from a National Geographic photo. Hard clay with sharp edged crevices and some embedded rocks for variety.
Shortly after entering I realized this was going to hurt. I have never tried to pull my fat arse across anything that caused me more pain that that stuff. Making matters worse was the fact that the house had been built on system of very sound footers. The kind that turn crawling to your probe more like finding the cheese in a maze. To go from one point to another (could only be 5' as the crow crawls) involved crawling all the way around these blockades.
I was experimenting with new crawling techniques like "The Sidewinder" (lying on my side and pulling myself along to give the pressure points on my knees a rest)before it was done.

This job would have taken maybe 3 hours at most, probably less, if I could have got up on my knees and boogied, but ended up taking 6 and left me totaled at the end. My knees, elbows and palms were sore to the touch and muscles groups I didn't know I had were commanding respect.