Which is the most reliable free antivirus for USB flash drives that resides and runs from the device itself and doesn't need to be installed on the computer?

I can think of two types of apps that could answer your question.

  1. Portable versions of an antivirus. These are only used to scan once the OS has booted. I'm going to call these trash since a rootkit can easliy evade this.
  2. Bootable Antivirus on USB. Now we're talking. Remember that no antivirus today is 100% effective and again we are talking SCAN ONLY as to be effective when the OS is running, we must install this to the OS.

So bootable AntiVirus (AV) scanners? Here's a list: https://www.geckoandfly.com/19582/free-usb-antivirus-fix-unbootable-windows-unmountable-boot-volume/
All these can be put onto USB or CD/DVD.

commented: what he said :-) +15

autorun antivirus is considered to be the most reliable.
but, it needs to be installed on PC

commented: That was helpful. NOT -4

autorun antivirus is considered to be the most reliable but, it needs to be installed on PC

A non-answer that doesn't even attempt to address the actual question asked by the OP. The actual answer, which does address the question, has already been given by rproffitt...

Check out WinClam - a free AV. There is also a Unix/MacOSX/Linux version available (called ClamAV). I use it all the time, and have never had a problem with it not detecting malware on my systems.

Avira PC Cleaner does the work for me, just a simple yet effective and user friendly antivirus.