Ooh... There are a few. I really like D&D 5e, but I wish there were more support for using a shield to hit people (something they were designed for - they should not be classed as improvised weapons) and for grappling. These are both things I feel Pathfinder did better. Likewise, I really like Pathfinder, but I fucking loathe feat trees and feel that feats that work like those in 5e would be much better.

I really like The Riddle of Steel, but the skill system is incredibly janky.

I really like GURPS, but I feel it could really use better guidance on how to use it for various settings.

The one I've run most often is Dungeon World, and while I think it is easily the best introduction to GMing on the market. The problem? No support whatsoever for PVP.

Beyond the Wall is also pretty awesome; the only problem I have with it is that it is written for more experienced players and GMs. A newbies guide (which would probably end up just looking like the GM section in a ptba game) would be really nice to see.