This post is a part of the #RPGaDAY series for 2017 by David F. Chapman and RPGBrigade. For more information, see this post at AUTOCRATIK. I'm modifying per suggestions from S. John Ross as well as applying my own interpretations. Comment with your answers or links to your own posts!
Day 15 - Which RPG do you enjoy adapting the most?
I interpreted this question by taking "adapt" to mean "hack" -- specifically "rules hack." "Adapting" could also refer just as easily to "reskinning," which is just as valid, but which doesn't give me much in the way of choosing one game over another. Reskinning, in my mind, should take advantage of a ruleset, certainly, or at least take it into account, but I don't find particular favor in reskinning one system over another by virtue of the system. I'm of the belief that the choice of a system for reskinning should be driven by what you're reskinning to. Rules hacking, however, implies a different set of criteria. The rules you are hacking definitely will affect the enjoyability of the hacking process.
This question also gave me a certain amount of pause, because upon some self-reflection, I realized that hacking games is not really something I do a lot! There are a million games out there, and I get a good experience out of many of them. If I find that a game experience isn't quite for me, it's often a lot easier and more productive for me to just find another game rather than try and hack up one that doesn't do it for me. I think the most common occurrence for this kind of thing for me, personally, is to bring the setting from one game into the system for another, but that's honestly just more reskinning if I don't drum up modifications to the second system. And even if I do, it's often small things or swapping in or out independent subsystems, basically just for the ease of not having to worry about design issues.
So. For this question, I'm going to wander into theoretical territory. If I were into adapting games, I think my system of choice would be Fate Core by Leonard Balsera, Brian Engard, Jeremy Keller, Ryan Macklin, and Mike Olson. This answer is potentially a bit of a cheat, not only because Fate Core proper is a generic system, but because it's purposely built to be modular and hackable. Those properties are exactly why I would find it an enjoyable system to hack, though. Fate certainly does promote, if not a genre, then a particular style or tone of play, in that it's geared for high, fast action and capable, heroic protagonists who are generally expected to succeed at things. These are qualities that I tend to enjoy in RPGs anyway, and so if I needed to adapt a system to run a game that looked like that, Fate would be my go-to. Also aside from being designed to be hackable, Fate hacking is also explicitly supported and encouraged by Evil Hat. For example, the Fate System Toolkit is effectively a manual on how to perform rules hacks on Fate Core, and I certainly enjoy hacking any system that tells me how to hack it. The kinds of games that Fate Core encourages, its design as a hackable system, and publisher-led support to hacking it all make it my top choice for today's answer.