For banking: Opera, best password management
For QA: Opera, vertical preview tabs, automatic reload
For development: Firefox, firebug, validator plugins (Opera has that too, but I don't like Dragonfly)
For surfing: Firefox, adblock, extensions
For mobile: Opera mini/mobile, quick and full view

I also have (tried) IE, Chrome, Epiphany, Skyfire, O3, Bolt and Lynx, but never use(d) them regularly.
As a last point: I like to try out new or old browsers, just to see what features I might be missing or from where we came. Just try Netscape 4.8 and you'll see how much that is.