Answer:
Unique product.
Step-by-step explanation:
The main driver of the LEGO's strategy is their unique product. Despite it is easily fakeble, LEGO has reach a level where the product is inseparable of the experience that offers the toy. Unlike, Hasbro or Mattel, unable yet, of produce a unique toy and an unique experience.
Some would say that is the marketing strategy the actual competitive advantage, but is important to remember that there is no good marketing strategy without a good product.