Chrome. Lighter and faster.
I am currently having a "software as a service" website built for me and I have to test it and report bugs and such to the developers. Therefore I have to use these other browsers. Explorer is still the slowest and the most bugs seem to occur using it. Like a "Save" button works fine in Safari, FF and Chrome, but doesn't function at all in Explorer. (??) I don't get it.
For me FF is bloated with extensions and stuff and Safari lacks some of the ease of use features that chrome has. Plus my Mac doesn't use a two button mouse. Which kinda sucks when you are used to one.