I never seem to pick one and stay with it, I alternate back and forth between FF, Chrome, and Opera, I use IE for somethings, played around with Safari when I accidentally let it install during an iTunes update, and even experimented with Flock.
I like FF the most, I don't use bookmark toolbar or anything that takes up extra space so it is the most slim and allows more screen for the page i'm looking at, plus all the extensions are great. I just use speed dial extension to give me the same effect as Opera or chrome's tab page.
I do like the new look of Opera, and am excited that FF4's first images show it as being similar.