>>44414096With the Tao dragon, without it it's debatable. As for Ghetsis, the narrative requires him to be the strongest in BW outside of N/Hildabert. As the narrative is more story driven, Alder serves as the "big good" meant to be a benchmark for how strong N had become. Ghetsis becomes the man behind the man after, and hence must be narrative-wise stronger.
Worth noting Ghetsis's Hydreigon at Lv.54 is capable of defeating Resh/Zek in battle, even though their "legendary" power renders them capable of destroying Unova.
"Strongest in the world" is also debatable. Blue claimed he was at one point, Giovanni was supposedly regarded (likely also a personal claim) such, Red was called "World champion," etc.
Overall, I think Ghetsis is up there, but the point is that it's a story implementation rather than something you're suppose to weigh against different narrative examples. Aside from the Orre games which is the only similar case, the other leaders weren't required to be end of campaign bosses in their regions.
