Chrome... im not loyal to it though, used to use FF.
A browsers UI is what sways me, IE's UI has always been garbage, turn on the "command bar" to see an example, shambles. Chrome and FF aren't really examples of doing it right either IMO because they lack the customization i would like to see, moving home buttons/back buttons, moving everything around to where i want it. No browser can do that though so chrome and FF are the best so far.
Im considering moving back to FF due to quite a few annoying things with chrome (someone already mentioned the gif thing). Middle mouse button behavior is erratic on some sites (works fine with IE).
EDIT: Just gave FF another shot and i found i can move pretty much everything around, forgot about that!