I'm going to disagree with just about everyone else on here. I'd say DW and similar force you to be a good GM, and don't really help you to do so (aside from some one-line, fortune cookie style words-to-live-by).
That said, I'm going to say one of the best assets for a mediocre GM for running a good game is good players. I've had many GMs that were railroady, or not very creative, or said no all the time to player creativity, and you know what? I still had fun in those games. Not as much as I would have if I had a better GM, but being an adult and just going with what you have in front of you is actually okay.
Now, for systems and settings that help the GM be a good GM, I would say the most important things are helping you to create content, and helping you to say yes to the players' creativity.
Helping you create content is a lot of times baked into the setting. Plot hooks and interesting NPCs and interesting locations are, for the most part, given in the description of the world. The best way to find a setting that does this for you is to just pick a setting you are really excited about. Those that have help with this outside of the setting usually do it in the form of tables full of ideas, which you can usually roll randomly on or pick one you like.
An example of a system that does this really well is [Stars Without Number](). You can create an entire sector of space, filled with detailed and interesting worlds, with alien races and interstellar-organizations, all in a matter of minutes (less if you use an online generator version). It also gives tools to run those organizations, playing a sort of GM game on the side.
Dungeons and Dragons usually does this pretty well too. The new DMG has lots of tables and charts to help generate content, and older versions of D&D have had them too.
The other important thing, having rules that help you say yes to the players, is where many editions of D&D have fallen short. Early on they weren't good at this because they simply had no guidance of any kind for outside-the-box ideas. Later editions weren't very good at this because they had so many rules that explicitly said no for you. The newest edition is pretty decent though, but still isn't really built for that kind of play.
This is also why Dungeon World is so praised. It nails this part so well, that it sort of sets the bar for that kind of play. Personally, I think it doesn't have anything else going for it, and there are other systems that do just as well, without forcing you to play that way. Fate is a little better (though not by much). Both games require great GMs more than they allow them.
Numenera, which someone else mentioned, and it's sister game The Strange, are at what I consider to be an ideal middle ground. They allow all the flexibility of DW or Fate, but have some structure there for you to lean on when you don't feel like having all the pressure on you in that moment (or if you are just not that great of a GM yet).