It depends on what you mean by "strongest".
Strongest by military? Diplomacy? Trade? Economy?
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Warlord as in the shichibukai? It is still unknown of who is the strongest or what all of their full abilities are. It is impossible to know who is strongest at this point in the story.
There are many Water Type Pokemon in Pokemon Platinum, however, there is only one strongest one. That Pokemon is Palkia. Palkia is one of the Main Storyline Legendaries in Pokemon Pearl, it is a Water/Dragon Type. If you are looking for the strongest Non-Legendary, your best bet is Gyarados. Any exceptions?
A Meditite,Machoke and strongest one in Lucario
The sage of six paths.
triangle is the strongest polygon