It is the best measure if the job requires extremely high intelligence. Only about one percent of jobs fall into this category.
One guy was absolutely brilliant. I don't know what his IQ was, but he was thought by everyone to be a genius (and was rumored to have an IQ to match). He was incredibly good at digging down into the most arcane issues and finding solutions to problems and identifying potential problems that might arise if certain actions were taken.
The other guy was just a good solid lawyer. Very smart, but not brilliant...just solid. He was very good in his understanding of the law, but was excellent when it came to the practical application of the law to the real life issues we were facing. I'm sure his IQ was quite high in that he was from a top school and had made partner at a young age at a top firm...but his gift was that he was balanced, measured, and mature. His gifts did not require nor would they have been expressed by a high IQ.
This is what I am referring to when I say that IQ alone isn't a good predictor of success (at least at the professional level of performance that I needed).