STL
The most common format. It holds geometry as triangles only — no units, colours or history. Export at high resolution so curves do not look faceted.
OBJ
Similar to STL but can carry colours and textures. Common from 3D scans and organic models.
3MF
A newer format that keeps units, multiple parts and materials in one file, which removes most scale confusion.
STEP
A CAD format with exact geometry. Best if we need to change dimensions, but it requires manual review.