Safari on the Mac, Firefox on Windows. I will add that I have been using Safari 3.1 on Windows XP recently as well with great success - it is finally quite close to the Mac version and may become my favorite.
I work in software development and use Windows at work only. I avoid IE there because it is slower and suffers from poor UI design - it is simply not well thought out. Also, as websites worldwide began to adopt open standards, IE simply does not render pages well and produces more page errors than Firefox or Safari, which conform more closely to
W3C guidelines.
Though Firefox supports more shear features, I love Safari on Mac OS X because it is direct, consistent and wickedly fast. That's what I'm on right now.