Which RPG do you enjoy adapting the most?
I have actually been adapting, or as we like to call it homebrewing, for decades now (wow, did I really just say decades?). Obviously D&D and its various editions have been adapted. Also I've done work with...
1E Gamma World (new monsters and mutations)
Masterbook (A superhero game)
Gumshoe (a fantasy version)
Numenera/Cypher System (superheroes)
Savage Worlds (Necessary Evil campaign with new powers, psionic campaign, fantasy world/campaign)
For awhile Savage Worlds was my go-to system for homebrewing. It is light and flexible with a solid foundation to build from. However, currently 5E D&D is my rpg of choice for adapting/homebrew. It is a system known by a large number of people. It is also light and flexible. I've adapted it for a modern version of Gamma World (kickstarter plug) and a science-fiction space-faring game. Both have been easy to do and people have said they are a lot of fun to play.