Firefox, but I have all major browsers to test my site. I also use Chrome for Google and 3dvia stuff, it seems Google Chrome is the best browser for plugins, but Firefox is useful for a lot too, because it's fast, has a good web console, and follows the web standards best. IE8, 7, 6 were terrible not only in UI but also for web developers. Poor Web Console, does NOT follow web standards, quite slow. But there are some things that REQUIRE you to use it, like Groove 3d Player from Otoy. It is from Microsoft after all, and who else knows their own product better? But with the addition of IE9, I start to like it more. Better security, the Javascript has been all brushed up on, and the CSS is absolutely wonderful. One problem still... IT IS SLOW! IE might be fast when it comes to things like browser tests, but I just did a test that showed I had to wait five seconds for my homepage (Babylon custom search page) to load and for me to able to click the search bar. Compare that to Firefox! Two seconds... less than half the time!
As for Safari, I can't judge because I'm on Windows, but it's slow for me. It will be faster for you because you might use a Mac, in which case it is faster.
And Opera? It's fast! But only because, while it is good with the web, it lacks some things that slow it down to make it go faster, that are also key features. In any other browser, if you hold a key down, it keeps firing. Not Opera! You must keep lousy variables between whether each key is down or up. And a lot of browser plugins simply aren't supported. See 3dvia Player! One of my most used plugins isn't supported. It may be fast, but it's not good for what I use it for.