Answer: Internet Explorer 6
According to The Atlantic Wire, "Vancouver-based AptiQuant's research was elegantly straightforward: it posted an IQ test on its website and looked at the IQ score and web browser of 101,326 people who completed the test."
While by no means scientific, AptiQuant's test did show that those who performed poorly on the test tended to use IE6. Those who scored the highest tended to use Opera.
Chad Vander Veen previously served as the editor of FutureStructure, and the associate editor of Government Technology and Public CIO magazines.