Microsoft IE [Win only]: It's installed with Windows. That should say it all. I haven't been bothered to use IE for quite a while. It takes up a lot of resources and feels slow. I know that there have been a lot of improvements on it but I don't really think it performs that well. The only reason I don't uninstall it is because there are some websites (usually local ones) that only display properly on IE.
Firefox [Win & Mac]: Firefox is the only browser (so far) that I use in both my PC and Mac. It seems to have the same performance on both OSes. It's faster than IE and it was the first browser I used with tabbed browsing (although all have followed suit). What really makes Firefox indispensable to me is its many addons. Stuff like downloads, saving websites, blocking ads, weather reports, tweeting, blogging, etc. etc. are made easier because of the addons. I think it's almost as fast as Safari (more on this later) but a bit slower than Chrome. What really bugs me about Firefox is that it seems to have poor memory management. After hours of use, it isn't unusual to see (via Task Manager [Win]; Activity Monitor [OSX]) it using up to 200+MB of memory even with just 1 tab open!
Safari [Mac only]: it's 64-bit although I really haven't noticed any performance difference. Its faster than Firefox BUT... it seems to load the whole webpage before displaying anything. Contrast that with Firefox which gradually displays parts of a page while it's loading. That alone makes a difference to me since I get to see stuff faster on Firefox even if the full loading of the page completes faster on Safari. It doesn't have addons either but there are work-arounds.
Chrome [Win only]: the newest and fastest of the browsers I've used. I also like its minimalistic interface - no distractions. This has taken the place of Firefox as my primary Windows' browser. Downside is, it doesn't have addons or extensions so there are quite a number of instances where I have to fire up Firefox. Chrome is the only browser I've used that runs tabs in its own memory space. Technically that means that if a webpage goes bad, only that tab should go down. Unlike Firefox and Safari where a badly programmed webpage could stall the whole browser. I haven't really been able to test this feature yet but of the 3 browsers, Chrome has the best memory management structure. I'm looking forward to test the Mac version of Chrome, which is still in alpha testing.