What is OFF?
OFF (Object File Format) is about as small as a mesh format gets - a header, a count of vertices and faces, then the numbers. It turns up mostly in academic geometry work and mesh-processing datasets, where being trivial to parse matters more than being widely supported.
What is GLB?
GLB is glTF packed into a single binary file - geometry, materials, textures and a node hierarchy in one blob. It is the format the web actually uses for 3D, and the one most viewers, AR tools and game engines want.