Firefox for me. I tried Chrome and didn't like it... Opera is good and I like the speed dial feature only slight drawback is you have to revert to Yahoo mail classic as it doesn't support newer version of Yahoo.
I was going to give Safari a whirl but was put off by the 19+MB download. Also the screenshot put me off with too much screen room given to buttons for iTunes etc.

Okay maybe it is customisable but right now Firefox zips along nicely so if it ain't broke don't fix it.