What massage gun attachments do I actually need?

The ball head (large round) handles 80% of use cases and is the best all around attachment for large and medium muscle groups. The flat head is useful for dense muscles like pecs and quads. The bullet/cone head targets specific trigger points and small muscles like forearms and feet. The fork head is designed for the spine area (paraspinal muscles). Most other specialty heads (air cushion, wedge) are rarely needed.