The Grand Line is filled with many strong Pirates, like Luffy himself. He has faced many enemies from the Grand Line, and even stronger in the New World.
Today, I decided to rank the strongest opponents that Luffy has come against.
5. Bartholomew Kuma
Bartholomew Kuma is one of the most integral members of the Revolutionary Army. He also faced the Strawhats at Sabaody Archipelago, where he absolutely overpowered each one of them. Like the rest of the crew, Luffy was also helpless against Kuma, since all his attacks were deflected, and were useless. Ultimately, he lost bitterly, and was sent flying to Amazon Lily.