YMMV, but for me it's Chrome. It seems to render the fastest and I like exactly how it is set up out of the box. The only modification I make is to add the MicroBlock extension, everything else is perfect. It is safer to use on sites with Flash or Java content as it uses its own software for that. Every tab is given its own sandbox which increases security.
Don't bother with Safari unless you are running Mac OSX, the Windows version of Safari is unsupported.