Depends on what you mean by best. The easiest way will be for the hotel to call a taxi for you. However, you can take bus, metro and train combinations (with transfers) which will be cheaper, not faster and certainly not easier. And, if you're carrying the usual 'cruise luggage', you won't want to drag that on the bus/metro/train... most likely. When I'm on vacation, I want to be on vacation.... so having the hotel arrange for a taxi would be my choice.
It's about 1 hour car ride from city due to traffic to the port for about ~ 80.00 - 100 Euros. A train from Marriott ~ 45 minutes, for 7.00 Euros to the port. Dealing with your bags to save $ will not be fun at all, TAXI service is your answer. Have a good cruise, enjoy the history.