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