Welcome to cruise-chat. Boy you ask a tough question. It really depends on how much time and energy you want to invest in the excursion and what other things you would like to do. With that said, here are my thoughts: First, Chaccoben is definitely "Mayan Ruins Lite" and the least impressive of the lot. Secondy, due to its proximity to Cancun and the numerous ships in port, Tulum is the most heavily visited - which also means it will be the MOST crowded. If you get there anytime after 9:00 AM, you will be sharing the views with about 80 other tour busses of visitors that all arrive at the same time and it is very HOT! As for the others: In Costa Maya, Kohunlich is probably the most impressive, but I believe that RC offers a combo tour of Kohunlich and Dzibanche which offers the best experience and diversity of exploring. If you are really into vsiting the ruins, that would be a good choice. Xunantunich is the largest and most impressive site in Belize. I know there are tour companies that offer a combined tour of Xunantunich and Cahal Pech (not worth it on its own) but not sure if it is offered through RC. I am not familiar with Oxtankah.
Now, as a side comment: There are so many other things to see and do on a first cruise - I would choose to visit the ruins in only one port. It is my opinion, that if you have seen one Mayan ruin, than you have seen them all. I am sure there are history and archaelogical buffs out there that would disagree - but that's my opinion and I'm sticking to it.