Personally I think that Firefox is the best choice right now.
Chrome is also very popular, but many people are suspicious about Google's tendency to grab and evaluate every bit of data it can get. Supposedly the browser has been made less intrusive over time, but I don't really trust it.
Opera was a great browser for years - efficient, innovative, very customizable. Since version 11 however, its quality is declining steadily. From version 12 onwards I had so many problems with it (for some reason some of my games would stutter and become unplayable while Opera was running in the background, closing Opera solved the problem immediately) that I finally switched to Firefox.
Internet Explorer is always available, and has much improved security now, but it's still a Microsoft product. Many people use it as a second option if their main browser doesn't work on a given page.
Why are you looking for alternatives to Firefox in the first place?
langurmonkey: Firefox has been running like shit lately. The performance is horrible. Explorer9 runs so much better but I don't want to use Explorer9.
Do you have any plugins active?
Firefox by itself is pretty fast, or at least not considerably slower than any other browser. Plugins are often programmed very inefficiently though.