Yes Ricky is the "strongest" opponent that Pac will have faced.
Ricky's physicality and sometimes reckless aggression will be unlike anything that Pacquaio will have faced before.

However, in opposition to the statement made by Nonito Donaire, he will not be technically the best body puncher that Pacquaio has faced. Erik Morales is.

With this fight, Hatton can win, but not with pure physicality, he must neutralise Pacquaio speed with his own movement and defence before thumping to the body.