##Opera
Opera
Because it's still my favourite browser. After having used it on Windows for eight years, I took it with me when I "switched" four years ago.
Browsers are always subjective, but I like Opera because I rely on its single-key shortcuts, used its mouse gestures a lot before I moved over to a glass trackpad, I have dozens of tabs open at any time, it has session handling, bookmark syncing, and dozens of other neat, small things. And everything is built in and just works.
I could probably get Firefox to do everything Opera does for me, but it would be a hassle, and it would be even slower than it already is without any extensions. I could never get Safari to do everything I need.