My personal opinion is Pirates of the Burning Seas. It has realistic sea combat (ala Sid Meier's Pirates!) and boarding combat and ground combat (ala EverQuest). The setting is unique since you wont find any other historical late 17th century MMORPG out there. In an age of fantasy MMORPG after fantasy MMORPG, PotBS is a refreshing and fun change.

If pirating and sailing is not your cup of tea, I suggest either Ultima Online or Dark Age of Camelot.