It's interesting to look back on Marineford , Garp really held back there also Sengoku probably saved Akainus life. Also the Red Haired pirates did have mercy on everyone, the Marines wouldn't have been able to stop them , Blackbeards crew wasn't fully formed yet either. I think power wise Shanks has the edge on Blackbeard especially with his Haki and Blackbeard seems to be especially reliant on devil fruits , Blackbeard strikes me as a Brawler just takes his opponents best shots , Law took the next step after beating Big Mom but Blackbeard was smart to ambush him before Law Fully grasped the experience he got fighting Big Mom and Kaido.