I do so love popularity contests! I think some people judge intelligence on the length of the posts . Still, the most intelligent poster to ever grace this board was by far AlanF. He had an amazingly logic way of reasoning through things. He'd beat pretty much anyone here in an IQ contest. I also put Hillary_Step up there.
AllTimeJeff is a contender. Also, SixofNine is like a darkhorse. I think it takes a great deal of intellect to be so consistently witty.
That being said, I happen to be in love with all of these guys, so I might be biased.