Tough One.

I Would Chose Either But I Would Have To Go With Megalodon Because It Is Much Larger Than The Dunkleosteus. Despite The Dunkleosteus Having Tough Armor, Megalodon Could Have Bitten Through Dunkleosteus Easily. And Megalodon Also Had Much More Bigger Speed So It Could Catch Up With The Dunkleosteus Easily.

NOTE: THEY DID NOT MEET! Megalodon First Appeared Around In The Miocene Era But Dunkleosteus First Appeared Around At The Devion Era.

The dunkleosteus lives in the warmer climates and was Thoth to swim in the shallow

no, it lived long ago

Dunkleosteus

probably whale would win