Hi Bivens, and welcome to cruise-chat.
I take this to mean you will be proposing on this cruise? Congratulations! The best cruise itinerary will be the one that you and your significant other will obviously enjoy. Do you envision proposing on the ship, on a beach, in one of the ports? And, as you posted this in the Princess forum, are you decided on Princess? (Good choice, BTW.)
Let us know more about itineraries and ships you're considering and a little bit about what you hope to get out of the cruise (other than a "yes!," of course), so we many provide further assistance.