Mostly Firefox. I have to force myself to open the others. I really want to give them a try. I am an (looks around to make sure Yrag isn't listening) add-on addict, so it's hard to give up FF.
I'm trying to use Chrome for work related stuff.
I haven't made up my mind about Opera yet. I was interedsted when it had Unite, but they have removed it for a while.
I tried Flock. Not for me.
I tried WYZO and may go back to that as it develops. It has some great potential.
Mozilla Labs is working on a new add-on for FF called SNOWL. This has a lot of potential for putting your feeds and messaging services all in one app that you can run in the sidebar or in the main screen. It's worth checking it out. Similar to that is a stand alone app from Mozilla called SPICEBIRD that I plan on checking out.