There is no single worst fear shared by every cat. Loud noises, unfamiliar animals, restraint, pain or sudden changes may trouble different individuals to different degrees. Look for what actually happens before your cat hides, freezes or tries to escape rather than testing it with things said to frighten cats online. Give it an accessible refuge and avoid pulling it out just to reassure it. Quiet company at a distance may be more helpful than forced contact. If the reaction is new, intense or accompanied by other changes, discuss it with your vet.
Physical discomfort can affect tolerance, and persistent fear may need professional behaviour support. The goal is not to make the cat confront every trigger. It is to reduce avoidable stress, preserve choices and help it feel secure in the parts of the home it uses. Keep a simple trigger record including noise, visitors, location and escape options; the same event may feel different when the cat has somewhere to retreat.
For product comparisons alongside this advice, see Velvet Nest and Sterling Court litter tray. The details below explain their sizes and uses.
Further reading: cats.org.uk; cats.org.uk; cats.org.uk; scottishspca.org.
Velvet Nest
Sterling Court litter tray
Taj-MeowHall