I have a Mac and my main browser is Safari. However, I've been trying Chrome now and again. Its a great browser dont get me wrong. I really want to like it, but its missing a few key things such as print preview of all things and better PDF support. It does a feel a tad smoother than Safari at times though.
I'm not a big extension guy so Safari meets my needs just fine. I also like its clean, minimalist interface and the way it manages tab as well as its tight integration with OS X. I just wish it wasn't such a memory hog.
Firefox downright sucks on Macs in my opinion.