What are your favorite games to mod?

Recently, I've gotten back into Skyrim for the umpteenth time since I got it for the Xbox 360 back in 2011. This game was a large motivator for me building my first PC years ago, due to the wealth of different mods available for it. Normally before each playthrough, I'll revise all my mods and add/remove a bunch to tailor the playthrough to whatever kind of character I'll be making. I'm currently running a stealth archer (very original, I know) with a modest 65 mods active. Fortunately, I have actually played more hours on this character than I spent modding in preparation. 

Other games that fall into this category, albeit to a lesser extent, include Oblivion, Fallout 3/New Vegas/4, and of course Minecraft.

What are some games that you have modded into oblivion? Or do you prefer to play your games vanilla?