This is based on my own subjective opinion. There is only one antagonist per arc. Below are the criteria I use in order of importance

1: Did they fight Luffy 1 on 1.

2: How much authority does the character have among those present in their faction.

3: How antagonistic is their relationship to Luffy.

4: How long did they fight Luffy

5: How strong was the character in their fight with Luffy, did they overpower or even defeat him?

Arcs are specified according to the wiki. Elbaph started on 1126 and will end when the wiki claims. Below is how I would have resolved this question for previous arcs:

Loguetown: Smoker

• Defeated Luffy in a fight

Reverse Mountain: Laboon

• Even though the relationship was not antagonistic Laboon was the only character with an extensive fight for Luffy.

• Mr 9 only "fought" Luffy for 1 panel.

Whisky Peak: Mr. 5

• Fought Luffy very briefly and was the highest ranked Baroque works operative.

Long Ring Long Land: Foxy

• Fought Luffy 1 on 1.

Water 7: Rob Lucci

• Defeated Luffy and is more antagonistic than Franky

Enies Lobby: Rob Lucci

• Very long fight.

Post-Ennies Lobby: Garp

• Highest ranked marine present.

Thriller Park: Gecko Moria

• Outranks Oars and fought Luffy 1 on 1 almost as long as Oars.

Sabaody Archipelago Arc: Sentomary

• Overpowered Luffy.

• Kizaru didn't fight Luffy and Kuma was rather saving the crew than fighting Luffy.

Marineford: Akainu

• Some characters fought Luffy more but Akainu is only outranked by Senguko and has a much more antagonistic relationship to Luffy.

• Overpowered Luffy so much that Ace got killed defending him.

Zou: Jack

• Highest ranked Beast pirate involved.

Whole Cake Island: Katakuri

• Longest fight.

Egghead: Kizaru

• Fought Luffy a few chapters 1 on 1.

• Higher ranked and stronger than Lucci.

• The Gorosei mostly fought Luffy as part of a brawl.

So if Luffy faces Shanks as part of a David Back fight or something, that could make him the main antagonist. However, if the arcs also contains a real fight against someone with a more antagonistic relationship, that is likely to take precedent.

Only one alternative will be resolved to Yes. In case the character is involved in multiple options, for example, the antagonist is Shanks that turn out to be the man marked by flames, and a god's knight and a marine, then only Shanks get resolved due to the rule of specificity. Shanks is more specific than man marked by flames which is more specific than god's knight which is more specific than marine as there are more marines than god's knights.

This is based on the manga and not other one piece media.