best antivirus for macbook air running Big Sur

None. Do not install third party antivirus apps. They are not needed, use system resources while providing no benefit, and they may interfere with built-in security.

macOS - Security - Apple

Set up your Mac to be secure - Apple Support

Protect your Mac from malware - Apple Support

Security features on your MacBook Pro - Apple Support

Effective defenses against malware and ot… - Apple Community

NONE period. Effective defence against malware and other threats If one what Malware defence Malwarebytes or Etrecheck

None. macOS has protection built in. All 3rd party antivirus will do is cause problems and consume resources.

Never mind the fact that there are no known viruses in the wild that can infect macOS.