Firefox 3.0.1. I like it's extensions, and that really stops me from switching to say Opera or Chrome. Memory load could be better though, and it crashes more often than it should.
I tried chrome, but it seemed to cause BSODing on my PC. And no extensions, or even basic stuff like not displaying images. And there have already been 2 exploits for it.
Safari, nope, I'm not installing Apple software on my PC, thanks.
IE 8 is OK, but not as many extensions, has compatibility issues, and seems slow, compared to Firefox. But otherwise, it's fine.