BM's butterfly knives are historically the knives most closely assiociated with them, but things have changed. Still, their line of balisongs deserve a mention, with the BM42 being the current standard. It is where the company came from, but balisongs have limited appeal and do not carry the company.
The AFCK propelled them to their current success and is one of their classic models. But it is no longer in production. It is one of their most known models, but it cannot be a flagship since it isn't sailing.
The 710 is their current flagship, at least in the bread and butter sense.
Normally a "flagship model" is the highest-end, most expensive, most advanced. A show-off for the brand. The 710 doesn't quite qualify. I'm not sure what their highest-end model is. But when I think of BM, I think of balisongs, the AFCK, and the 710. In that order chronologically, not in terms of importance.
My favorite BM models are the 630 Skirmish and 610 RUKUS.
The Griptilian deserves a mention. It isn't their high-end knife, but it has mass appeal. It is their Ford Focus or Mustang. While Ford's true flagship is the Ford GT.