I love Mozilla Firefox, it's fast, got many plug-ins and it can be used with any platform (almost).
But i must say that i was really disappointed when i found out that the only Android phone that it does not run on, is HTC Hero, the phone i own :-(
But Chrome is good to, its simple and fast.
I has never ever used Safari. I have never like Apple's attempt to become a monopoly...
Opera is really fast to, but not all sites works with Opera...