Favorite: Creosote, its sticky and tar like, very difficult to remediate, but with careful engineering and adequate data you can manage it. I had a site in London, UK where they had a former manufactured gas plant (MGP), which is similar to creosote (it was coal tar instead) and because it it denser than water it will slowly seep into the subsurface and stop on the nearest impermeable layer. Which in this case was a unit called the London Clay layer. We conducted soil borings throughout the site and determined the topographic surface of the clay layer beneath the site and identified a low point where the coal tar would naturally settle. We then installed a well at that low point and began recovering the coal tar through the well with specialized remediation equipment.

Least Favorite: tetrachloroethylene, its everywhere, sticks around in the environment forever, and it degrades into even worse chemicals, including vinyl chloride over time.