Answer:
Well, it doesn't really matter on which Bakugan Element is the strongest. It's the skills and strategies that you use to fight the opponent. But yeah, it's
Pyrus - Fire
Aquos - Water
Subterra - Earth
Darkus - Dark
Haos - Light
Ventus - Wind
Pyrus vs. Aquos
Subterra vs. Ventus
Darkus vs. Haos
One has a disadvantage on their opponents Battlefield.
Depending on the Gate Card, even when trying to power up one's own element, you might unfortunately increase the opponent's even more.