It'll pretty much depend what you do with them. So far FF4 is working very well and fast for me, I also like Chrome which is fast (I tend to run Chrome with no add ons at all for speed and FF with add ons that I like to use for work &c). IE I find terribly slow & clunnk & I only use it where I have too - s9ome sites prefer it)
Benchmark battle: Chrome vs. IE vs. Firefox | The Download Blog - Download.com