Generally the most popular are my history posts:

On the Ancient Egyptian Value for Pi

On the Ancient Babylonian Value for Pi

My personal favorites:

What is Algebra? (and why you might have the wrong idea and why it is important)

Nonlinear lesson design (Giant Ants of Doom)

The design of mathematical notation

Proof of the Fundamental Theorem of Arithmetic (with fancy design)

The winners for most controversial would be my series on how multiplication is not repeated addition:

The “Multiplication is Not Repeated Addition” Research

“Multiplication is Not Repeated Addition” Revisited

The Educational Problem with Teaching Multiplication as Repeated Addition