We don't actually know that a single Lord was stronger than the mightiest of everlasting dragons. It may have been that the Lords couldn't have won without such superior numbers. Gough mentions how 'for every one of them, we lost three score of our own,' and this indicates that there, then, must have been a LOT more units in the Lord's armies. How strong a single Lord, then, is compared to a single, high-tier everlasting dragon is actually up to question. The Lords never even fought Kalameet, and no one ever says that Kalameet is even the absolute strongest dragon; there may others that are even fiercer.
That being said, I vote Tarkus.