I think Apple would say it's Safari...
Having said that I think it comes down to which browser is best for the user rather than the platform.
Personally I've used Firefox for years, and had no problems at all. I guess it might depends on what you are asking of the browser? Do you need a load of plug-ins and add-ons?
A lot of others will be surprised I use Firefox because they have a favourite of their own or have had bad experiences.
Switched to Chrome a while back and haven't used another browser since. There are a few niggling annoyances with it though (not being able to 'hard refresh' pages without going through a menu to empty the cache for example), though this only really affects me when I'm altering a web design.