rayzoredge

To be literal, you are the safest Internet browser.
 
To mitigate online threats, you have to avoid risky online behavior. This includes but is not limited to: downloading possibly-infected files (which, basically, is inclusive of anything you download from the Internet), visiting possibly-hazardous sites with malicious code, looking into spam e-mails, getting phished, and not browsing with security deterrents (router which provides an internal firewall, software firewall, etc.). You also have to watch out for exploits (security holes) that malicious code and hackers can penetrate to gain access to your machine and/or its resources. This is where your operating systems and browsers come in, which everyone has seemed to suggest.
 
If you want complete security, go with a Linux-based operating system. The reasoning behind this is a robust community that continuously works to update and improve the operating system, keeping you up-to-date with security, features, and other goodies. It also isn't as targeted for attack because of the fact that Linux holds a very small audience with operating systems compared with the alternative.
 
You could also look into Apple's OSX, which is the second-best for security. OSX is actually the least secure out of all of the popular operating systems available, but as with Linux, it does not have as much of a market share in operating systems as the ubiquitous Windows platform, which makes it secure in the fact that there isn't a lot of threats specifically made for a small audience. Makes sense, right?
 
Last but not least, we have Windows. Each version of Windows will have its own security flaws and strengths, but keeping up to date with system updates and patches will help prevent the many attacks, exploits, holes, and malware that Windows is infamously plagued with. It is imperative that with a Microsoft OS, you utilize a firewall in conjunction with anti-malware/anti-virus software and whatever else you may deem necessary.
 
For browsers, it seems that Microsoft Internet Explorer is the large target with its large number of discovered exploits and ActiveX vulnerabilities. Everyone guns for IE because everyone else seems to use it. For this reason alone, you should avoid using Internet Explorer.
 
However, the use of Mozilla Firefox is growing, and I'm sure that in the future, there will be more guns pointed in this direction. But Mozilla has more than a handful of features to throw back and keep the guns at bay. With extensions like NoScript and built-in security features, along with a robust community and continuous improvements and updates, Firefox may be the most popular and considerably-secure browser to use today.
 
Opera, Safari, Chrome, and other browsers are still rather low-key as far as market share, so they may or may not be the safest out of the major key players. Like with operating systems, the guns are pointed at the more popular choices, so you are probably good to go. They all seem to have good development, and Safari has the native advantage of being available for OSX, which already houses the security advantages I outlined above. Chrome may actually become a larger player just because of Google's name behind the project, but I believe that it will have the benefits of gradual adoption along with fewer attacks as Firefox had when it emerged to compete with Internet Explorer. Opera, as good of a browser as it is, hasn't garnered as much of an audience as its fans would like, but maybe that's a good thing.
 
I am currently using Mozilla Firefox with NoScript, Windows XP SP3, and Windows Firewall on my work computer and the same but with SP2 and Norton Internet Security 2009 on my home laptop and I believe that combination of software should suffice for most users. Feel free to play around with other combinations, but just keep in mind that the end-user is the one line of defense that could render all of these security deterrents useless.

No antivirus is 100% effective. Nowadays, more of the focus is generalized around malware, to include spyware, Trojans, and other malicious code. 
Personally, I run Norton Internet Security Suite 2009 to give myself some sort of an illusion that my PC is secure. I also utilize Panda's scan services and Malwarebyte's Anti-Malware for full manual system scans just to make sure... and even then, I'm not 100% satisfied.
 
It still boils down to individual usage habits. If you indulge in risky behavior or peruse in popular attack venues (like social networking, sharing files with others, and even Internet shopping), you can fall for a lot of socially-engineered attacks, let a virus or Trojan into your system, etc.