With One Piece's Wano arc in the Manga now over and the Egghead Island arc underway, many fans will be aware of Monkey D. Luffy's rapid growth in terms of raw strength and jump in the aspect of his Gomu Gomu No Mi Devil Fruit now known as the Hito Hito No Mi. Recent developments in the story revealed the true properties of the fruit, and alongside Luffy's development in learning how to infuse Conqueror's Haki into regular attacks in the midst of the battle with Kaido, it stands as Luffy's Gear 5th.
Given the One Piece story reaching closer to its climax and taking into account Luffy being the main character, it is very much reasonable to suggest Luffy at the very least could give the toughest in the series, a run for their money.
Warning: Manga spoilers ahead!
5 Kizaru
After Luffy's long and tough encounter with Kaido and coming out as the victor, it was more than enough reason for him to become one of the new Yonko. A large majority of fans believe Yonkos are far stronger than Admirals, especially after chapter 1055 which left Greenbull leaving Wano after catching wind of the Red Hair Pirates' presence once Shanks emanated his Conqueror's Haki. Going by the beast and deadly nature of Kaido, who was dubbed the strongest creature in the world of One Piece, Admirals such as Kizaru would almost certainly struggle against Luffy who defeated Kaido.
Fans will know that the series is in the endgame now, and if Luffy is to be held back by the likes of Kizaru with the aspiration of becoming pirate king, then it would not make much sense. That is not to say that Kizaru is any pushover, but the huge gap in strength should be clear now.
4 Trafalgar D. Water Law
While it is highly unlikely for the two of them to duel given their strong relationship and the alliance with the Straw Hats from the Punk Hazard arc until the end of Wano, Law would most certainly find it tough to defeat Luffy let alone commit any real damage. The Ope Ope No Mi stands as one of the strongest and most useful Devil Fruits in the entirety of One Piece, given its combat capabilities and the option of giving another immortality. Given Luffy's Gear 5 awakening allowing him to use the environment to full advantage and the level of freedom it contains in battle, many would assume Luffy would defeat Law in combat without any doubts.
This comes with the suggestion that Luffy has always been stronger than Law, based on Luffy being the only one capable of defeating Doflamingo among the two at Dressrosa. While the pair have definitely come a long way since then, Luffy's growth is arguably the most exponential among all of the Worst Generation.
3 Mihawk
With Luffy's encounter with Mihawk way back in Marineford pre-time skip ending with Luffy needing to escape his attacks, many may assume that this time around, things would most certainly be different. While Luffy's weakness is still sword and slashing attacks as revealed by Kaido in chapter 1047, it would be unreasonable to assume that he could not give Mihawk a difficult time.
Mihawk is said to be close to the same level as Shanks and is even dubbed as the greatest swordsman, so to say Luffy would defeat Mihawk is still debatable in the eyes of many fans. However, with Luffy's advancement in Armament Haki and the ability to deflect incoming attacks using Gear 5, Luffy may be one of the very few characters in One Piece who would offer a big challenge to Mihawk at this point in the story.
2 Marshall D. Teach - Blackbeard
Standing as presumably the direct antagonist of the series and the final enemy for Luffy to face, Blackbeard is arguably one of if not the strongest characters in the world of One Piece. This is simply due to the number of Devil Fruit abilities he has, with one of them having the ability to nullify others' Devil Fruit abilities. Whilst fans are still yet to see the full capabilities of the Hito Hito No Mi Model Nika, the combination of the Fruit's abilities and the newfound prominence in Haki, Luffy is one of the few characters that could put Blackbeard on the back foot.
It would most certainly be a tough encounter to call and seeing how this fight will play out would be interesting, to say the least, due to the Yami Yami No Mi. Since many would assume that Luffy is much more versed in his use of Haki with the infusion of Conqueror's in attacks, this in itself may very well nullify the Yami Yami No Mi, as Haki supposedly supersedes all Devil Fruit abilities.
1 Eustass Kid
If there is any character among the Worst Generation who would struggle the most up against Monkey D. Luffy, it would be Eustass Kid. While he certainly does contain levels of strength that are comparable to those in the top tier, as he has the same bounty as Luffy and Law, he would almost definitely struggle with Luffy's Gear 5 and most likely lose.
Despite taking down Big Mom alongside Law, this does not compare to Luffy's capability of going 'mano a mano' with Kaido in base form before his awakening. Aside from the brute force attacks, many would argue that Kid does not pack a punch anywhere near the level of Luffy and perhaps even Zoro, so he would most certainly have a tough task up against Luffy.