Firefox 3.5 definitely. Its speed, security, and features are good, but the real draw for me is addons. I have the top five browsers installed (IE 8, FF 3.5, Opera 10, Chrome 3, and Safari 4), but can't stand to use anything other than Firefox with Adblock Plus installed for very long. I mainly use the others to test out my webpages. I think Chrome would be a distant second for me if I had to choose because of its speed and the stability it has from running each tab in a separate process.