Strictly speaking of IQ it's likely INTP. Research seems to suggest that thinkers have a higher average number of gifted individuals than related feeler types, like INTPs compared to INFPs, but both of those types in particular have a high percentage of gifted individuals.
Here is a chart.
And a link for a discussion of why.
Which Is The Smartest MBTI Type? | Slayerment
Keep in mind that people from all personality types can be gifted, but we're talking about averages. Also, real world success and IQ aren't the same. Extroverts with high emotional intelligence and a positive attitude tend to have higher paying jobs on average.
MBTI Socioeconomic Infographic | Career Assessment Site
One reason INTJs may seem like the smartest type, and they are up there, is because INTPs don't apply themselves to real world accomplishments as often, so you have a higher number of visible INTJ intellectuals. ISTPs are a nice mix of outgoing and competitive, with Ti dominance, so again you get a higher number of public figures, like Steve Jobs, who many think was an ISTP.