IE9 now (and in fact, IE going back to the beginning). It does everything I want/need, is the recommended browser for certain sites I visit (and I've tested and discovered why), I answer IE questions and as a general rule of thumb prefer to use products for which I'm going to offer advice {except for advice not to use} or recommend (and that extends beyond browsers to anything).
In that vein, for testing or for helping with minor issues/questions and to remain aware of the competition, I also have (and keep current) and sometimes use briefly for a change of pace and to update favorites and add-ons and to maintain a marginal background familiarity (and other such things), Firefox (the "normal" current version - none of the many variants I just heard about), Chrome, Opera, and Safari. I was surprised to read about the others here and have since discovered far more - and while tempting, I need to draw the line somewhere and believe these are the most commonly (if not exclusively) used ones in the forums where I do most of my work.
To be honest, I wish I could mix-and-match the best features in each of the above into "my" browser <alas> - and every choice comes with positives and negatives - but excluding some options which shouldn't even be considered or aren't applicable to my versions of Windows, I believe most of those mentioned are perfectly fine candidates and the choice depends a great deal of what one does with the computer, the computer OS and specifications, the network bandwidth, and of course, habit, familiarity, personal preferences, and compatibility with how and where it is used. And in work environments, especially domain-based, sometimes one has no choice in the matter on work computers and, if remote access if possible, chances are high that use of that same browser would be required on the remote computer (as in if you work from home and want to access the company network).
I do, however, recommend installing at least a second option if the primary is actually the only one installed at least for backup and testing (but do remember to keep it current just like all other programs on the system regardless of how often they are used if only for security purposes). Secunia PSI
http://secunia.com/vulnerability_scanning/personal/ can help a great deal in notifications and in quite a few cases even automatic updating not only browsers but many if not most other installed programs on the computer.