Pacquiao definitely stunned Floyd with his straight left.. He hurt Floyd worse than Canelo ever managed.
It's a different type of power. Pacquiao probably has more snap and speed on his punches but Canelo's might have more force behind it. Skills also play a role because it's how they land that counts also.
Shane Mosley was trading shots with Canelo but was **** scared against Pac. That tells you all you need to know about their respective punching powers.