I voted for Opera, but it's really a tie between Opera and Firefox.
I really dig the fast, lightweight, clean interface of Opera. I've also used some of Opera's Unite add-on apps (with mixed results) and it has the best pop-up ad blocker I've used (this works well on Tom's). Opera's 'Mini" browser is easily my preferred mobile browser.
My problem with Opera is that I can't use it alone. Many websites just don't format correctly in Opera, including ver. 11. Opera's Unite apps are also very buggy and have stolen many hours of my life troubleshooting.
Firefox is like the old reliable pick-up truck you keep parked on the street that you can always count on when your sleek new ride is being a b*tch.
Oh, and FYI, I'm a Mac user but I rarely run Safari on any of my three Macintosh systems.