After being beaten by Kaido in Kuri, Luffy learned to use Ryou and Advanced Conqueror's Haki. With his newly learned abilities, Luffy was finally able to injure Kaido in a battle.
In this formidable show of strength, we are at least made known that he was defeated 7 times, and captured by the marines a couple of times as well.
Luffy defeating Kaidō has been one of the biggest milestones of his career as a pirate. It officially made him one of the Four Emperors and established him as one of the strongest living characters in the world of One Piece.
Garp Is Considered To Be One Of The Strongest Marines. Also renowned as 'Garp The Hero' and 'Garp The Fist', Monkey D. Garp is a Vice-Admiral of the Marines. Being their strongest known member in history, there's no denying that Garp is capable of taking on Kaido.
1) Kaido. Kaido did in fact lose to Luffy's Gear 5 at the end of the Wano Country arc. However, keep in mind that he withstood multiple waves of strong enemies before this final encounter. Luffy was also completely worn out after the fight and fell into a coma for about a week.
Zoro manages to cut Kaido!
The Oni are a rather mysterious race as almost nothing is known about them. They haven't been officially introduced in the series yet, but there is no doubt that they will be confirmed as a separate race, sooner rather than later. There are two known members of this race- Kaido and Yamato.
Kozuki Oden and Roronoa Zoro both gave him permanent scars with their swords attacks. Keep in mind that neighter fighters have unlocked the ability to forge "black blades." Mihawk should be strong enough to scar Kaido as well. It should also be noted that Kaido considers Shanks to be his equal.
The Luffy who started the fight against Kaido wouldn't be able to defeat him, no. Without any help, Luffy would've lost at this point. He had no chance against Kaido, he learned Advanced Haoshoku Haki, but it was very shallow.
He even fought against Captain Rocks on God Valley. Garp may be old right now, however, he is still more than capable of taking on Shanks in battle. This is evident from the fact that he recently set out to fight not just Blackbeard, but his entire crew on his own so as to rescue Koby from the grasp of the Yonko.
3 Magellan
Magellan is the Vice-Warden of Impel Down and before the time skip, he held the position of the Warden. He clashed against Luffy in the fourth level of Impel Down, known as the Burning Hell. Magellan was way stronger than Luffy and was able to easily overpower him in battle.
King the Conflagration is one of the three All-Stars of the Beasts Pirates and Kaidou's right-hand man. Born as Alber, he is a member of the near-extinct lunarian race from the Red Line, noted for their ability to create fire.
Chapter 1040 saw the momentous defeat of one of these emperors, Charlotte Linlin, better known by her epithet "Big Mom". Big Mom's defeat by Luffy's allies (for the time being) Trafalgar Law and Eustass Kid, has major implications for the future of One Piece.
Concerning his ethnic appearance, Oda revealed that he imagines Zoro to be of Japanese descent, albeit in a real-world context.
After his death, Yamato chose to become just like Oden, impersonating him and adopting his mannerisms. As a result of this desire to emulate Oden, Yamato decided to become a man and identify as Kaidou's son, and Kaidou and his crew refer to him as such as well.
Flame Dragon Torch is, without a doubt, Kaido at his very strongest.
One Piece 1049: Luffy Triumphs Over Kaido.
The man known to be the strongest creature, Kaido is widely known to be the most powerful pirate right now, and the mightiest of all Yonko currently. He was an apprentice on Rocks D. Xebec's ship long ago and he was given his Devil Fruit by Big Mom on God Valley. Since then, Kaido has continued to grow stronger.
Ussop would never leave his village. Zoro would either be executed or he would escape and be a lone fugitive searching for Mihawk. He is the only one who might end up going to the grandline.
I don't think you can simply say "he can beat an admiral" because even within the Admirals, there's such a huge difference in power. I personally think that Luffy could probably just barely beat Aokoji now (or in the near future), and he could fight but not beat Kizaru, and has no chance of beating Akainu.
Gear 10th Luffy is Luffy's ultimate form from the upcoming speculative fan-made sequel to One Piece, Son of the Pirate King. Luffy has continuously trained his haki and Devil Fruit powers during his time as Pirate King until he achieved this gear.