To me, there would be a difference between "most popular" and "most built". Popularity can be measured in how visible a locomotive is, if it became the face of a railroad, etc.
If in terms of popularity, I'm going to vote for the 4-4-0 "American Standard" style. It shows up on images used for lots of things relating to trains, was responsible for most of the motive power during the growth of the US, the wheel arrangement was even revered in Europe for general use.
Just behind that I would say a 4-6-2 "Pacific". Used for premier passenger trains around the world, seen by a lot of people and quite a few railroads put a lot of PR material out using their Pacifics as the example.