wait, people actually use IE?? and firefox ftw, meets CSS standards and stuff unlike IE, pfft @ microsoft
The most compliant at the moment is probably Opera 9 because Firefox has quite a few things that need to be fixed. I think with Firefox 3 it will become as good or better. At the moment I use Firefox on the basis that I love the customizability such as themes and addons, and scripts that come into play in addons and I am used to the layout now more than any other browser.