In theory, the idea of a game where you can go wherever you want, and do whatever you want, is the formula for the perfect videogame. In practice, this takes a tonne of work to pull off, and frankly, many developers do not have the time, resources, or skill to pull this off, so lots of open world games kind of fall flat.
For me, there are two elements that are crucial to making an enjoyable open world.
1) There needs to be interesting traversal.
2) There needs to be interesting content to discover.
For traversal, if the main method of travel isn't fun, then I am probably going to just zone out. The problem is, having a fun method to explore the map is really quite difficult to pull off. The obvious success in this area is Spider-Man (PS4). Moving around this city is just so much fun, that when I first started playing, I just spent about an hour swinging around the city, before I even interacted with my first objective. Conversely, when I played Red Dead Redemption 2, once the awe of the environments wore off, when I was travelling from objective to objective, I would just hold X on my controller as my horse automatically rode to where I needed to be, whilst I was scrolling Twitter for minutes at a time.
Then there needs to be interesting content to find. If there is no reward for exploring, then I am not going to explore. A game like The Witcher 3 is famous for its side-content: Each quest is interesting in some way, with a bespoke story being told for each one - some are even more interesting than the main quest. On the other hand, when you have something like Far Cry, where there are dozens of generic bases to capture, or dozens of generic bounties to collect, I might do one or two, but after that, I will probably just ignore the rest. If your content is lazy, and there is no reason to travel off the beaten path, then the open world effectively just turns into one elaborate loading screen for each main mission.
How do you bring these things together, then? Getting the content right is probably the easier part of the two, but traversal is much more difficult, because its not like you can be a superhero in every videogame. I don't feel like fast travel is much of a solution, because if you are just teleporting from waypoint to waypoint, there may as well be no content in between. To me, the the curse here is scale. If you have a massive map, but no interesting way to get around it, it is probably going to cause glazed eyes. Instead, rein it in; make a smaller, denser area, like a small town or city district. If you don't have to travel miles to get from one side of the map to the other, it doesn't really matter that the best speed that you can muster is a slight jog.
To me, the closest an open-world game got to being "perfect" were the Batman Arkham games. Gliding, grappling, and the Batmobile are fun ways to get around the city (which also isn't massive), and the different supervillains that you encounter in the game's side content are fun to discover.
There are gems with problems, though. As mentioned before, The Witcher 3 is great, despite it being quite slow to get around the map. Spider-Man (PS4) is a fantastic superhero power fantasy, but I would probably just skip all of the sidemissions. And Bethesda games do the whole "look in any direction, and you can go there" gameplay loop better than anyone else, but the content isn't all that interesting, either.
For the most part though, I generally roll my eyes at a new open-world game. I think so much gets sacrificed in pursuit of scale, that it normally isn't worth it in the end.