Based on the North American episodes I would rank them...
Pikachu
Snivy
Swadloon
Oshawott
Tepig
Tranquil
Scraggy
I honestly don't know where to rank Tranquil, it has yet to battle and even as a Pidove it didn't do much battling either. I don't think Oshawott is that far in strength compare to Tepig, but he has main character power-ups that lets him gain wins against stronger pokemon. It's better to be weak and win than to be weak and lose.
Based on what I've heard Palpitoad is ranked at around Snivy/Swadloon level and Roggenrola is around Oshawott/Tepig/Tranquil level
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