I'm not going with a basic answer like Roblox or Minecraft. (Even though I said MC in 2021)
My favorite game is Star Trek: Bridge Commander.
The game lets you play space battles as different ships from the Star Trek series. You can be any ship you want and fight anyone you want. The game is really old, but people develop lots of mods to add new features. It's always cool to see which ship class is the strongest...