The best browser is the one the user gets on best with.
I use a variety depending on the sites I browse and the hardware:
Firefox4 - Main browser I use on my desktop PC. It works fine with pretty much every website I can think of.
Internet Explorer 8 - I use at work for compatibility testing, e.g. when making changes to a website. My work PC has XP, so I can only use Internet Explorer 9 in Virtual PC.
Internet Explorer 9 - I tend to use it as a second browser when I need to be logged on under two accounts, e.g. my blog to check standard user view when updating plug-ins, etc. I also had a few cases where it crashed the PC, i.e. could not bring up the Task manger to kill its process, thus requiring a hard-reboot, probably a compatibility/bug issue with my graphics card with its new acceleration features.
Chrome - Main browser I use on my Netbook. It seems to run quicker and smoother than Firefox, likely due to the limited Atom CPU power. In the past, I mainly switched for its better viewing area, although Firefox4 can now have the equivalent viewing area by disabling the bookmarks toolbar and any other toolbars.
Opera - I use Opera Mini as the main browser on my mobile phone as it loads pages a lot quicker than the built-in web browser. However, on my PC, I use the Opera browser for testing purposes only.
Safari - Occasional testing purposes only. Even though I rarely use it, occasionally it just dies for no apparent reason, requiring it to be reinstalled. The installation of a Quicktime alternative update seems to be the main culprit for killing the Safari installation.