I use Mozilla currently but plan to eventually migrate to Firefox. I've tried Opera in the past and didn't particularly care for it. I hear it's great, but it just doesn't fit me.
Aside from the fact IE has been lagging far behind feature-wise for a while, I don't like its integration with the OS and the inherent security issues. It's a lot better than it used to be, but there's still too much potential for security issues. That goes for anything that uses IE's engine.