Before Firefox 3, Safari and Konqueror were my favorite. They had a smaller memory footprint and were a lot faster in rendering.
With Firefox 3, that changed and now I used Fx3 as my primary browser. Still I believe the future is WebKit and not Gecko, that's why every time I'm behind a Windows box I use Chrome.
The things that matter to me in a browser are rendering speed and standard compliance.
Opera is pretty good on the standard compliance part, but when it comes to rendering I find it slower than other browsers. Maybe not rendering, but overall I find Opera a bit sluggish.

If only Microsoft will IE to more standard compliance, IE8 is a step forward but still not there yet.