I'm glad you made this thread, because I've been wanting to address this thing since the spoilers, little throw back to my huge discussion with Hannibal.
When it comes to Pirate King, there are two components that make one the Pirate King:
1. Reaching Raftel & Conquering the grandline
2. Being the strongest pirate on the seas, as freedom as a pirate lays in strength
This is something that Monkey D. Luffy will showcase as the 2nd Pirate King , and something that Gol D. Roger already showcased as the first and only Pirate King.
Reaching Raftel & Conquering the grandline:
- Mentioned by Luffy when he saw the red line before Saboady
- Mentioned by Rayleigh that Roger became known as the PK after they completed the grandline
- Mentioned many other times as well
Being the strongest pirate on the seas:
Whitebeard, Big Mom, more than likely Shiki, Kaido, and countless other souls possessed Conqueror's Haki. However Gol D. Roger considered to be the one who stands above them. This is not possible if Roger didn't display things that made the world of OP think "Ok damn this dude is above Whitebeard". So standing above rest of the pirates with CoC is something that is needed to be considered the PK by fellow pirates.
Shiki viewed Roger as a wall for himself & others, he did not view Whitebeard as a wall. This statement holding even greater weight now as he had long connection with Whitebeard, thus had info on his ambitions and what he was aiming for.
Sengoku, at the time an admiral of the marines mentioning the title of Pirate King as a title of power:
Prior to that Shiki mentions how the marines could've never captured Roger, because he knew how strong he was:
Now we can take a look at Luffy, to him he won't be the PK if he can't beat the Yonko & Admirals.
All in all the title of Pirate King is a symbol/representation of one's superiority on the seas as the top pirate fighter. Being superior to Whitebeard being no exception. Whether in reality he was superior to Whitebeard or not is a whole different discussion that will revolve around the interpretation of 2 panels in the manga. However, the world of One Piece viewed Gol D. Roger > Whitebeard, veteran pirates like Don Chinjao viewed Gol D. Roger > Whitebeard. And of course this wouldn't be the case if there was nothing backing it up.
So the Pirate King is viewed upon as the strongest pirate on the seas. And I emphasis "strongest PIRATE", because in parallel to the rise of Gol D. Roger, we also had the rise of Monkey D. Garp, the superiority between these two is unclear.
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The titles that should be in question whether they have a connection to being 1v1 strongest are WSM & WSC. Neither of these titles having any connection to being the strongest fighter.