Level Up/A5e, hands down. It takes the 5e ruleset and makes adjustments to add more granularity, to mechanically enable options that I feel could use it, and to provide more versatility and choice in the rules to players and GMs. And it does all this while maintaining the 5e core with which so many potential players are familiar.
I can't think of a single change A5e made to vanilla 5e that wasn't some degree of improvement from my point of view. Highlights include the increased granularity to the skill system (skill specialties and expertise/hindrance dice), the origin system (heritage, culture, background, destiny for character creation), and the journey system (a huge expansion of exploration and social options and mechanics), among many others.
In addition, A5e has an amazing community of 3pp content creators, many of which worked or work on the core A5e game, who actually are aware of and work with each other to support each other's content, which does a great job of maintaining consistency. The creativity in these folks is astonishing. And of course the core game keeps expanding too, with new supplements (in new genres even), and a great monthly periodical in the Gate Pass Gazette, soon to be joined by another focusing on the science fiction themes of the recently released Voidrunner's Codex.
It is an absolutely phenomenal game. On a personal note, I have been unhappy with the current state and direction of WotC's version of the 5e ruleset for some time, and I feel the presence of Level Up has maintained my interest in the active hobby to a degree that wouldn't have happened otherwise. I'm so thankful for that.