You displayed a depth of knowledge in your initial interviews that is extremely impressive.
Explanation
"We are happy to let you know that we that we think you're pretty clever." Is informal and based on this message it does not fit.
"Our intention at this juncture is to scrutinize your intellectual capacity and creative prowess" Is very words and seems a bit excessive. It doesn't match other parts of the assignment.
"You seem way smarter than a lot of people." Doesn't fit the formality at all.
Therefore "You displayed a depth of knowledge in you initial interviews that is extremely impressive" is the best way to go.
