One Piece has become near synonymous with the idea of the never-ending story. However, even timeless journeys like the Straw Hats' must eventually come to an end, and it wasn't too long ago that the series would get an actual goal. Once the Straw Hats found the giant elephant island of Zou, they discovered the means of finding the final island, Laugh Tale: Find four, red poneglyphs known as the Road Poneglyphs.
By reading the information on them, one would be able to identify the exact location of Laugh Tale and, thus, the One Piece. The location of three have already been revealed— one on Zou, another on Whole Cake Island, and a third on Wano— but fans are still waiting to discover where the last one is.
10 It's Being Guarded By The Marines
The World Government and the Marines have some of the highest stakes regarding the race to the Pirate King's throne. They already have their hands full with the Great Pirate Era that Gol D. Roger started and have been worse for wear after Whitebeard's proclamation. If fan theories are to be believed, and Laugh Tale and the One Piece do have some kind of weapon or incriminating information against the World Government, then finding them wouldn't just inspire a new generation of pirates but also threaten the status quo of the powers that be.
Knowing this, it would make sense that the World Government and the Marines would do everything in their power to keep people from finding Laugh Tale, including finding one of the Road Poneglyphs themselves and hiding it away in some vault.
9 Mariejois
Mariejois is the capital of the World Government and is the homeland to the Celestial Dragons. Since the One Piece is both a great treasure as well as a massive threat to the their authority, it's likely that the World Nobles themselves would want to keep a piece of the puzzle to themselves.
The secret of the Void Century and the missing country do directly relate to them; and if there is some dirt on the Poneglyphs and within Laugh Tale, they'd want to keep every piece of information as close to them as possible. This means hiding it from every outsider possible, including the rulers of the other kingdoms and even the Marines.
8 Fishman Island
During Kozuki Oden's backstory, it was revealed that the missing Road Poneglyph was once on Fishman Island, right next to Joy Boy's message. However, given that the Road Poneglyph wasn't there when the Straw Hats arrived, it was likely taken or moved.
Given that Fishman Island has plenty of ancient secrets beneath its belt, such as the Tamatebako and Noah, it would make sense that the Ryugu Kingdom would also try to hide and protect the last Road Poneglyph, especially given the sudden rise of pirates during the Great Pirate Era. This could give greater importance and tension towards the fishmen, especially if it ties to Madame Shyarly's prediction about Luffy.
7 Sphinx
Whitebeard was one of Gol D. Roger's closest peers and someone he could likely trust to guard the last Road Ponelgyph. The two did have a heartfelt conversation before Roger's arrest where the newly crowned king apparently revealed the story of the Will of D. Roger could've told Newgate at time about the One Piece and entrusted him to protect the last poneglyph.
Whitebeard before was also the protector of Fishman Island, the poneglyphs prior known location. It makes sense that Whitebeard himself, waiting and judging for the next, true king, would've protected the Road Poneglyph and maybe even hid it within his secret village on Sphinx.
6 Elbaph
Much like Wano, Elbaph is another powerful country within the world of One Piece that the story has been teasing for ages. The Straw Hats may likely visit the land of giants soon and, for the sake of narrative efficiency, may coincidentally find the last Road Poneglyph there.
Either because Roger entrusted it to them or because they found it themselves, the giants would be ideal protectors for the last Road Poneglyph. Besides Mother Caramel's seemingly defunct camp, Elbaph doesn't have any known connections to the World Government; and if anyone did learn that they were keeping the final clue to Laugh Tale, few people would jump to take it from them.
5 It's With Captain John's Treasure
Captain John initially debuted as one of the general zombies on Thriller Bark whose famous treasure has been Buggy the Clown's main goal since entering the Grand Line. He was apparently one of the legendary names under the Rocks Pirates and had quite a propensity for greed, keeping a mass treasure to himself and eventually getting killed by his own crew for it.
It would be interesting if the great treasure that he coveted so much included the missing Road Poneglyph. This would add as to why Buggy, the only member of the Roger Pirates to not go to Laugh Tale personally, would want to hunt down John's treasure so badly.
4 The All Blue
In One Piece: Stampede, it was revealed that a few members of Roger's crew had an Eternal Pose made so that they could find Laugh Tale again later. However, not wanting to ruin the adventure for anyone, Roger threw the easiest means of finding the One Piece into the ocean. With that in mind, it was easy for plenty of fans to guess that Roger might've done the same to the last Road Poneglyph.
It would be better and even more narratively efficient if he sunk the Road Poneglyph within the All Blue. This would give some much needed weight to Sanji's dream and an opportunity for the Straw Hats to have another, underwater adventure.
3 It's With Blackbeard
Given his status as the series' main villain, it would be appropriate if Marshall D. Teach had already found the last Road Poneglyph and would have to protect it in a grand battle against the Straw Hats.
Blackbeard does seem to be aware of the Will of D. and Roger's legacy which could've given him some clue as to where the poneglyph was; and if Whitebeard was protecting the Road Poneglyph, Blackbeard would've likely also found it when he was claiming Whitebeard's islands as his own. Keeping it on some hidden location or personally protecting it on Hochinosu, Blackbeard may really be Luffy's final test before earning the title of King of the Pirates.
2 It's Copied On Luffy's Straw Hat
This one would arguably upset some fans for its cliché-ness if it were true, but Oda has been putting greater thematic and narrative emphasis on the accessory as of late. Luffy's Straw Hat having the last Road Poneglyph doesn't necessarily have to be literal (though stranger things have happened in the series), but it could have the poneglyph's information written on it.
In a last ditch effort to protect his greatest treasure, Roger may have disposed of or hidden the last poneglyph for good and left the only remains of its information written on his prized possession, likely within the straw hats' red band. This would mean that inheriting the straw hat isn't just inheriting the will of the previous wearer but the actual right to become King of the Pirates.
1 It's With Shanks
As one of the most active, powerful pirates around and possibly even Roger's own successor, "Red Haired" Shanks could currently be protecting the last Road Poneglyph. Shanks does have prior knowledge of its location and may very well be the person who moved it long ago.
He's also one of the most peaceful and pragmatic characters in the series, meaning that he'd be the ideal person to guard the poneglyph from both power, hungry nobles and greedy pirates. This would put the Red Haired Pirates at the center of the major conflicts to come, especially when it comes to Luffy finally meeting Shanks again and, possibly even more importantly, Shanks' long-awaited battle with Blackbeard.