I used to be 100% FireFox.
But after using Safari more and more since version 4, I have realized it just loads super fast, starts up super fast, and doesn't render pages incorrectly, at all, anymore.
100% Safari now, with Chrome as alternative, and FireFox + Firebug during web development and web design.
Opera is always a disappointment to me. Lots of promises, little delivery.
IE is only used in Vmware situations to test web design and development, so I can then spend an hour figuring out if it's my code or a bug with IE *again*.