It's a tough one now that IE10 came out. I really enjoy the improvements that have been released along with Firefox. Their dev team does a really good job in my opinion keeping things up to date and secure. I think it's important to note that they were the first ones with patches against a few exploits that affected multiple browsers not too long ago, and the browser was also one of the only ones that was protected against a recent HTML5 data storage "bug". I would use IE as my main browser, but I have made it a routine to be using some of the Firefox addon's that I have, and no other browser has addons that can compete unfortunately. Firefox is still #1 when it comes down to extensibility. Opera did well on the security side of things for a while, but I haven't kept tabs on them for a bit, so I don't know how they would all compare at this point in time. There's a few idiosyncrasies though in Opera and Chrome that I do and don't like, along with IE and the download popup that can't be disabled as far as I know.
Chrome is the only one that can drag and drop shortcuts from active links, which I do like, in case I don't want to bookmark... But IE seems to provide a much smoother browsing session on a few things, so I also like IE for a few minor things.
I switch around from time to time though.
