I use Camino most of the time. Then trade off between Shiira, Safari, and Firefox the rest.
Safari is the defacto standard and is very fast. I have a few problems with certain pages but nothing awful.
Shiira is fun and uses webkit. The last release has been less than stable for me though.
Firefox (I use the optimized Deerpark version) Is also fast, though painfuly slow to start up. It is not as fast as Safari IMHO but it is still quite good. Extensions are a great thing though, giving you all sorts of options.
Camino has a native feel to it and is fast. As fast as Safari for me, maybe a bit faster. It does not support Firefox extensions though, which is a shame. I am not sure exactly why I use Camino to be honest. I switch my primary browser occasionally but always come back to Camino.
I use Quicksilver, Ma.gnolia, and ListMixer for bookmarks so swapping between browsers does not really bother me.