I am not sure which one is the best. I have always used Travel Guard and find that for us it is the best. Needed the medical coverage on one trip and it was great. They paid a lot more than the fee we paid.
There are quite a few to choose from, just check them all out and find the best one for you and your traveling companions. We've been using CSA for the past few cruises and have had no problems, but then again, have not had to make a claim. Here are are few sites you can look at:
We have always used Travelguard. Recently, I used insuremytrip to compare prices and features of each insurance policy. One comparison really surprised me, the company could not sell insurance to residents of my home state! Travelguard still was the best value for the most comprehensive coverage. It even would cover me for "unemployment" which could be an issue in 2003 for me. Our coverage cost was $127 per couple for cruise & air up to $1500pp.
I checked out Travelguard.com and they only offer $250 for canceling just for changing your mind. Celebrity offers 75% of the non-refundable portion as a credit towards another cruise. Sounds like a decent offer, but as someone else stated, purchasing the travel insurance through the cruiseline you're traveling on may have other risks.