It really depends on what you want out of a browser. If you're a Web developer, you may want to use Firefox because it has the most tools and add-ons. The
many thousands of available add-ons also make for a very robust browsing experience for people who spend a lot of time doing a lot of things on the internet. Updates to add-ons are announced within the browser as soon as they become available, so things tend to work all of the time.
Chrome is going to be a great browser. It is fast, nice to look at, has great features and has a growing extensions library. It also runs on WebKit which is the engine that powers Safari. Excellent with RAM usage.
Safari is the best for fast, vanilla (well, chocolate chip) Web browsing. Very few features, slim pickings with add-ons and extensions. Simple and crisp.
Opera is truly the Mercedes-Benz of browsers. Way too many features for most people, very complicated to learn and use to the fullest extent. It does weird things to markup, CSS and Javascript sometimes, but if you can get it going, is a pleasure to drive.
One last thing to consider: Firefox and Safari are notorious consumers of RAM. If you have a lot of RAM, this won't be a problem, but for extended sessions with default RAM packages on the Mac, you may find yourself needing to quit and restart your browser several times a day. I don't use Safari very much, so not very familiar with its features, but Firefox, at least, has a Restart Firefox item in the File menu, which clears the memory it's been using and re-opens all tabs and windows automatically. A clumsy workaround, but at least it's available.
My suggestion is to give all of these browsers a go and see what you feel comfortable with, keeping in mind that default installations in all of them can be customized and added onto.
Note:
Firefox 4 is now at beta 6. It's really amazing, but of course, there are still a few glitches. Definitely worth looking at. If you install it alongside a Firefox 3.6.10 (latest release) version, make sure you keep the two versions in different locations inside your Applications folder so that the earlier version isn't over-written.