Which RPG do you enjoy using as is?
Pretty much anything that's running on the Cypher System, and Numenera in particular. It's light enough that the rules don't get in the way of the fun, but still have enough systems in it to make your decisions interesting. It's also my favorite system to run as a GM (closely followed by D&D 5e). I've lifted the GM Intrusion mechanic and dropped it into other systems because it really fits with my style. It's a great system that I love playing.