Safari.
Few reasons.
1. color rendering is true to image. View a website in safari and in firefox at the same time. You will see Safari's as being much more vivid. From a design standpoint, it is more accurate to how the web designer intended.
2. HTML5/ CSS3 is already up and working pleasant.
3. It gives you awesome details on what exactly of a site isn't working properly and lets you easily turn off cache, java, flash, and anything else you may want to do.
4. The history can be really useful when you forget what site it was that you found that really awesome thing you wanted.
5. instead of increasing text size for difficult to read sites, it allows you to zoom so that all images, text and everything zooms and the page isn't actually distorted.
6. If you are on a mac, safari uses the Macs native spell checker so you always know when you are spelling something wrong.
Id say the color rendering, spell check, and zoom are the reasons that I'm sold on safari.
I do prefer Firefox's firebug web developing tools over Safari's though... thats about it.
