>>37895781Because it's consistent, ambitious AND well-executed.
Consistent because the entire line and its evolutions signal a concept and the progression of that concept: A lowly thief becoming a master thief. Though many Base Forms are simple and often do not manifest the theme clearly, as
>>37899061 stated, even Froakie pulls off the lowly japanese thief from the start. Frogadier is a progression from that lowly position: Better built and more agile. Ninjas are considered the master thieves in most media, especially RPG's so Greninja is the logical peak. Add to that Japan's association of frogs with Ninjas because of old folklore and you got yourself a winner.
Ambitious because while other lines try something simple and excel at it (like the Totodile line) a few other try too many things and don't manage to make them work. The Froakie line is one of the few that handles a concept and it does so wonderfully, taking various risks that feel in character and consistent with the correspondent animal instead of going for a less ambitious and safer route, which leads to
Execution. Nailing a simple theme is preferable to failing a complex one. Fortunately Greninja, as stated previously, has both the concept going for it AND the well done execution. The tongue scarf is one of my favorite design choices ever, because it really helps balance the design in a clever and believable way, adding a touch of silliness that evens something that could easily go overboard in the edge department, making it appeal to nearly every kind of crowd.
Torterra is still my favorite fully evolved Starter, but the Froakie line is definitely the most consistently amazing line overall. They're fantastic.