Firefox and Opera.
Although on any sites with adverts or involving typing, Firefox. (Adblock and inline spell check.)
I also prefer Firefox's page search feature, and miss the 'go' button in Opera.
I like the ability in Opera to create a new tab with a right click. I don't think there's an easy way in Firefox- I always end up opening a bookmark in a new tab.
OK I just found you can drag a 'new tab' button to the toolbar: now Firefox is perfect.
EDIT: I like the search feature in Opera 9.21 more, I just tried it: great improvement, although I still prefer the search pane at the bottom.