Question

Which European country raised the biggest threat to Spanish dominance in the New world and why

Answer 1

The British did because they had a large fleet and control over the colonies.All it takes is for British ships to defeat a Spanish Armada.
Then the Spanish trade would be disrupted and Britain would still have control

Answer 2

The Dutch became a commercially powerful rival to Spain--Amsterdam soon became trade hub for all the Atlantic World. They distinguished themselves as commercial leaders in the seventeenth century, as their mode of colonization relied on powerful corporations: the Dutch East India Company, chartered in 1602 to trade in Asia, and the Dutch West India Company, established in 1621 to colonize and trade in the Americas.