I like all three (Safari, Firefox, Chrome), but use Firefox the most. In my subjective opinion, here are the strengths/weaknesses that matter most to me:
Firefox is the ugliest, despite the availability of so many themes (most of which are even uglier than the default)
Chrome is the most aesthetically pleasing, including some of the visual cues like when it downloads files and opens and closes new tabs.
Firefox with the smooth scrolling option set has the smoothest scrolling when using a non-Apple mouse's scroll wheel (the rest are jerky, although Chrome has some add-ons that help). Smooth scrolling isn't just nicer looking, it feels easier on my eyes.
I don't notice any speed differences between the three but I mostly just go to text and image sites. But Firefox is the fastest at going "back" to the previous page--usually instantaneous, whereas both Safari and Chrome frequently seem to reload stuff from the server before displaying the previous page.
I use different computers and different OSs. Until recently, only Firefox kept my bookmarks in sync across all computers, and it's still the only one that keeps my passwords in sync (via the Xmarks add-on)