Opera rules my online world - no other browser even comes close.
It's the most user-centric browser, giving you far more detailed control over what websites are and are not allowed to do (you can even set display and behavioral preferences on a per-site basis), and giving absolute control over the UI. With Opera you can make the browser look and behave exactly how you want - button by button, field by field, image by script.
It has more comprehensive status information, more thorough script debugging, more sophisticated management of bookmarks, better keyboard navigation, built in user scripting, built in mouse gestures, easy options for things like disabling JS and images that you can put right on the toolbar.
And it handles history navigation properly - when you click "back" in Opera's history it doesn't reload the previous page, it literally re-creates its previous processor snapshot, which is invaluable for form-based interfaces, not to mention speeding up average load times by optimizing caching behavior.
And those "cool new" features in Firefox, like restoring a session when restarting the browser, and drag/restore of tabs - Opera's had them for years!