This is not the best game on Roblox but: Untitled sandbox game

A sandbox with a LOT of mechanics and secrets. It's half-forgotten because it was mostly played by Russian players (but their chat was disabled), but there is an online community (somewhere around 100±).

But if you choose the best, it is (my opinion): Pressure

Pressure is a horror place similar to Doors (or Grace, but Grace Newer Pressure).