This is a worthwhile read:
http://www.anti-malware-test.com/?q=node/180
The report is a year out of date so the detection rate tests may have changed.
All Anti Virus programs are limited in their ability to find infections in direct relation to the Author's ability to keep the software up to date. As the report states there are a huge number of new infections on the internet every day so it is an ongoing battle for any Anti Virus Author to keep their software updated. This equates to any system being vulnerable no matter what Anti Virus software you are using as new infections are only known about once they have been found which is too late to stop some PC's being infected.
The most dangerous infections 'Rootkits' are designed to be undetectable and can be in your system, sometimes for many months, before you have any clue they are there. They will open a Backdoor to your system allowing a hacker to have full access to all your personal data. In some cases, especially with Backdoor infections, the advice is often to Format & Reinstall the OS, once a system is compromised with this type of infection it can never be fully trusted again.
If you look into any Malware thread you will see tools used to find Rootkits and other Malware that are specifically designed to detect the infections and then other tools are used to remove them. You won't see a standard Anti Virus program used for these purposes. Special training is required to learn how to use these tools as in the wrong hands you can easily make your system unusable.
In conclusion, there is "No best Anti Virus" and it is impossible to give any PC a 100% cast iron protection against infection. Every time you update your Anti Virus there will be new infections out there that your Anti Virus will not detect. It is a game of cat & mouse and the Malware Author's will always be one step ahead.
All you can do is make sure whatever you use that the software is kept up to date and you never turn off Windows Update.
Secunia PSI is a FREE security tool designed to detect vulnerable and out-dated programs and plug-ins which expose your PC to attacks. Attacks exploiting vulnerable programs and plug-ins are rarely blocked by traditional anti-virus and are therefore increasingly "popular" among criminals.
I am approaching the advanced stage in a Malware course that will qualify me to join the Malware Staff. Anyone can sign up for the training which is long and hard, you will be amazed what you will learn about Malware, too much for me to write into this thread. The courses are not designed just so you can protect your own system, they are designed so you can help on the forums, so don't jump into a course unless you are willing to become a helper.