Look, this is pretty easy to settle.
The overall majority of browsers used is Internet Explorer just because it ships with Windows from every edition since Windows 95, and it is a very simple-to-use interface, unfortunately also very vulnerable.
Firefox is quickly catching on because of it's market share and advertising. It really is just a bogged-down version of Mozilla Suite (now SeaMonkey, or on Macs, Camino), but it runs well when configured both manually and with a program such as FireTune or Fasterfox. It also has support for many plugins, extensions, and such, that most browsers don't have. It recieves most of the recognition of main-day browsers because of the fact that it is knocking Internet Explorer down a peg, though it really has done very little damage at this point, only taking about 8-10% of the market share, meaning IE still makes up about 85-90% of the market share, or more. Of course, the big thing for developers is that it's open-source, so that's why so many plugins and extensions are available.
Netscape Browser 8 is really just a Firefox code bogged down with useless things that have always pertained to Netscape, since the days of Netscape Navigator and Netscape Communicator.
Avant Browser is basically an all-in-one suite version of IE, with less security vulnerabilities, RoboForm, and tabbed browsing as a default setting, not needing a toolbar addon. It also has a limited configureable interface, though the looks don't vary very much.
Opera is by far the most underrated of them all, and has long been the innovator of such things as tabbed browsing, unfortunately of course it was shareware and adfilled up until their most recent version, Opera 8.5. In fact, I would go as far as saying had it not been shareware and not been filled with advertisements for most of its history, as well as not being available to the public until version 2.0, it would be ahead of Firefox in the market share, and probably would have a large chunk of the share. It was one of the first browsers with voice support and Voice XML support, DOM support, cross-platform support, mouse gestures, total keyboard support (meaning you don't even need to use the mouse in order to control the browser, if you configure it correctly), page zooming, SSR, "fit-to-screen", MDI, cascading browser windows, session saving, popup blocking built-in, CSS, SVG, and mobile browsers. It is partnering with Google for its mobile browser, and is making the web browser for the Nintendo DS. Unfortunately Opera's overall global share of the browser market as being between 0.5% and 0.9%, but is big in Europe because it's a European-based company.
When Flock finally makes a stable nonbeta release, as i've been anticipating for months, i'll be migrating to that over Firefox and Opera.
K-Meleon is also commonly used due to its lightening-fast speeds and very little use of eye candy.
Overall, if Konqueror was available to everyone, and not just users of Gnome OS, we'd all be a lot better off, because Konqueror is by far the best browser out there today, unfortunately being limited to one platform.