Boy it's really tough to vote.
If I'm playing a pirate, the pirate troops, especially the French Renegade are great for bording, being both good at muskets AND melee.
If I'm playing a naval commander or field captain, English troops with their better pure muskets rule.
If I'm constantly attacking and sieging, Spain's Sword N Board infantry are great for pushing into a fort.
If I'm roleplaying a slave trader, Dutch cav is great for their mobility in order to more quickly attack and then scoop up prisoners from native parties.