Actually, the defaulty out of the box is something called Safari but I find most peoplle split into two camps.
The converts like me tend to go towards IE as it seems to be the most forgiving of non-standard code although most of the tricks like the colored scroll bars don't work.
The lifers tend to go with either the Mozilla option or in the case of the new OS, the built in browser.
The real trouble tends to be that none of them support any wide bunch of protocols so you end up with IE handeling DHTML better than Mozilla but not as much CSS which Mozilla seems to do better with.
I have on my system for the puropse of testing for Mac compatability (I'm a little biased but then most of the higher end graphics people, and I'm not generalizing here, there are always acceptions, etc etc etc, use macs):
Opera
Firebird
Camino
IE
Mozilla
Safari
Netscape
Firefox
All are Mac OS based browsers. Camino, Firefox and Firebird are Mozilla offshoots. As it turns out, according to my web stats, the majority of Mac users that visit are using IE or Safari.
So if you code to fit them, you'll be doing well.