The "most" open world game is going to differ based on what you are looking for when you say, "most open world." If you are strictly talking about the size of the open world, then something like "No Man's Sky" (not a F2P game) could probably be considered one of the most open world games. EVE Online (which does offer a limited free access point) would also fit in this definition as both games offer galaxies of “open world” to explore. If by the "most" open world you mean the most freedom to go wherever you want to go, then perhaps games like Minecraft or Rust are the options you are looking f

There aren't a ton of truly open world games in the free-to-play MMO game space, but games like EVE Online, Albion Online, and Planetside 2 offer some type of open world component to their gameplay. Often you may see games advertised as open world (like Tower of Fantasy or Genshin Impact) and to a certain extent they are, but sometimes level gating is in use to keep players out of areas where the content may be harder than the player’s character level. You'll also often see games in the free-to-play genre referred to as "sandbox" and this usually means the game has some type of "do whatever yo