Trafalgar Law is the Captain of the Heart Pirates and one of the members of the Worst Generation, making him a big player in the Great Pirate Era. With a bounty of 200 million berries, Law arrived on Sabaody Archipelago as an infamous pirate from North Blue.
After the time-skip in One Piece, his bounty has shot up to 500 million berries, proving that he truly is an elite pirate of the sea. Law has been central to the Four Emperors Saga of the series and has made great improvement in skill since then.
10 10. Luffy
Luffy is a member of the Worst Generation, and quite possibly the strongest of the lot after Blackbeard. He has a bounty of 1.5 billion berries on his head which has earned him the title of the "Fifth Emperor of the Sea."
With the powers of the Gomu Gomu no Mi and an incredible Haki, Luffy is levels above Trafalgar Law for now.
9 Marshall D. Teach
The strongest of the Worst Generation, Marshall D. Teach is a former member of the Whitebeard Pirates who defected from the ship to form the Blackbeard Pirates.
Over the years, he has gone on to become a Yonko of the Sea and even defeated the likes of the Whitebeard Pirates remnants. Teach, being a Yonko, is in a league of his own which Trafalgar Law isn't even close to just yet.
8 Donquixote Doflamingo
One of the former Warlords of the Sea, Donquixote Doflamingo was quite a ferocious pirate prior to his fall at the hands of Monkey D. Luffy. He possessed great skill as a pirate and his strength was displayed to the fans throughout the Dressrosa arc.
Trafalgar Law, who was quite powerful himself, was unable to take down Doflamingo in combat, which just goes to show that Doflamingo was head and shoulders above him.
7 King
The right-hand man of Kaido, King the Wildfire is a powerful Calamity who possesses tremendous skill as a pirate. Although his bounty is unknown, it is likely much above the 1.32 billion berries mark which is held by Queen the Plague.
King has the powers of the Ancient Zoan Pteranodon Devil Fruit which lets him transform into the creature and its hybrid as well. While his full strength hasn't been seen yet, King is definitely stronger than Trafalgar Law.
6 Benn Beckman
The First Mate of the Red-Hair Pirates, Benn Beckman is an elite pirate whose skills in the crew are second only to Shanks himself. Benn is likely a very capable Haki user, as are all the other powerful known Vice Captains in the series.
Kizaru openly acknowledged his skills during the Marineford arc of One Piece and Brannew also mentioned that Benn Beckman's bounty is very high, indicating that he's at least as strong as the other Yonko right-hand men, if not stronger.
5 Charlotte Katakuri
Katakuri is the right-hand man of the Yonko Charlotte Linlin and has a bounty of 1.057 billion berries on his head. He is a user of the Special Paramecia Mochi Mochi no Mi, and all the three Haki types.
Katakuri can use two advanced Haki types in Armament and Observation. His skills with Observation Haki, in particular, are extraordinary and allow him to see into the future itself. Without a doubt, Katakuri is much stronger than Trafalgar Law.
4 Marco
Famed throughout the New World as "The Phoenix," Marco is the former right-hand man of Yonko Whitebeard and the First Division Commander of the Whitebeard Pirates. During the war of Marineford, Marco fought and matched all three Admirals of the Navy, which is a testament to his strength as a pirate.
His fight against Kizaru, in particular, proved that he was more than a match for him. Trafalgar Law, while strong, is nowhere close to his level of skill just yet.
3 Silvers Rayleigh
Rayleigh is the former Vice-Captain of the Roger Pirates and the man who was said to be the right arm of the Pirate King. Famed as the Dark King, Rayleigh is immensely powerful. Even in his old age, Garp considered him a legendary threat that the Navy had to deal with carefully.
Kizaru, despite being an Admiral, openly admitted that he wasn't capable of catching him alone. Rayleigh, even in his old age, is several times stronger than Law.
2 Borsalino
Also known as Marine Admiral Kizaru, Borsalino is one of the strongest characters in the series, which isn't much of surprise since he is an Admiral. He is a proficient user of two Haki types and also has the Logia type Pika Pika no Mi to aid him in battle.
Before the time-skip, Kizaru destroyed four Supernova Captains without any trouble whatsoever. Although Law can hold his own against him for a while, he's still not quite on his level yet.
1 Kaido
Kaido is a Yonko who is said to be the strongest creature on land, air, and sea. His strength is largely considered to be unmatched in the One Piece world. Kaido powers are so majestic that even Luffy, a man worth 1.5 billion berries, wasn't able to graze him in combat.
His Haki, along with his Mythical Zoan Dragon Devil Fruit, makes him impossible to fight, except for those who possess the same title as him.