Not power levels always, but more like ideals and personalities clash:
Upper Middle matchups: Teach and Luffy
Opposing ideals for the pirates, how they see things:
Personalities are opposite:
Upper Left and right matchups: Buggy and Shanks
Another opposing ideals for the pirates being selfless and selfish, opposing characters.
Lower middle matchups: Koby and Imu
Opposing ideals for Government, Koby is probably the most selfless and honest government officer while Imu represents the most corrupt and selfish side of the government.
Lower left and right: Sabo and Akainu
Sabo as revolutionary does not discriminate meanwhile Akainu kills innocent people and kills people for their bloodline:
Also kills a Marine who was begging for his family:
Akainu's one of the most heinous acts was bombarding a ship full of innocent people and children:
And Sabo when he was a child was also a victim of a similar heinous act:
Lower middle left right: Aokiji and Dragon
Former Marines who tried to change the system from inside, and they failed, and they left, and they were both trained by Garp. But they choose different paths after leaving.
Who both search for freedom, as Oda said in SBS Aokiji joined BB pirates for living freely and Dragon also couldn't find any freedom and justice in Marines as well so he established something different after leaving them. Interesting that if its coincidence.
The only thing we don't know about Garling and Shamrock, perhaps Oda will also make something out of this.