Which is the best antivirus software for Mac, in your opinion?
OS X.
There are no known viruses for OS X; Trojans, such as this, require the user to actually do something, typically accomplished through misdirection or trickery.
The best anti-malware for any platform is between the chair and the keyboard. This trojan, for example, is an IM or e-mail purportedly from a friend; it then points to a fake YouTube video, actually a Java applet, which then requires an admin password to install. It’s what hackers call “social engineering.”
That said, ClamXav is free, robust, free, frequently updated, and free. I run it every few months just to make sure I haven’t picked up a Windows virus I might unwittingly pass on. It has yet to find anything.