Which RPG do you enjoy adapting the most?

I don't really "adapt" games that much these days as there are a ton of them out there nowadays and I can usually find one I like that covers what I want to do. If you want to broaden the question to using a system for multiple different types of games then I will say "Savage Worlds" - it's a great fit for a lot of genres and a good-enough fit for almost all of the rest. I've run and played different genres in GURPS, Hero, and d20 and SW beats them all in my opinion if I have to pick one system to run every possible kind of game. From cowboys to pirates to knights to jedi to juicers it works and works well in real, actual play. It always seems to be the "next" game on the list so we don't play it as much as I wish we did but it's a great game system and has been for a long time.