Day #16’s question was “Which RPG do you enjoy using as is?”
I guess I enjoy most games as is. It’s nice to experience games as the designer intended them. If a game is impossible to enjoy without hacking it, it is either fundamentally broken, badly puts across the designers intent of play or is designed to be hacked from the start.
I’m not sure there are any games I prefer to play straight over some custom variant or without any house-rules whatsoever, except possibly Blades in the Dark (and that is possibly because I’ve never needed house-rules, or felt the need to hack it yet), Psi*Run (it is so simple it really needs nothing changing), Fiasco (because again, it’s a highly focused, very simple experience) and other similar things. I almost play Numenera straight, but I swap out the armour rules for the ones in the generic Cypher System rules instead, generally.