>>25070545Greninja is popular because:
- It's a final stage starter.
- It's a fricking ninja.
- Ash has a ~special~ one.
The fact that it's broken to some degree or another in 6v6 singles isn't really a factor in its popularity.
If you replace 'ninja' with 'dragon', Charizard prior to Gen 6 fits all of this while being fucking terrible competitively - they then intentionally 'overpowered' Charizard to match the fact that it's popular.