The short answer: the one you have and use regularly is the best one.
The long answer…
There are pluses and minuses to all of them. The most important factor to remember is that anti-virus software is only effective if you use it regularly. If you’re constantly bypassing it and haven’t set up regular scans, nothing is going to help protect your machine.
Personally, I really like ClamAV. It’s a open source project, meaning it’s built by people who are personally interested in it, the source code is available for anyone to look at and it’s free. It comes in a number of flavors to match your operating system (such as ClamWin for Windows and ClamXav for Macs.) It’s easy to set up, unobtrusive and highly configurable.
There are dozens of options out there, just pick one and use it!