Personally, I like Travel Guard the best -- it covers nearly anything that can happen, including pre-existing medical conditions. And, you'll find one of our advertisers in CruiseMates' classified area -- The Travel Concierge -- offers steep discounts on Travel Guard and others.
I've used them all and their coverage is pretty comparable, but I've never yet had to make a claim so it's hard to say how they rate when you really need their service.
Just remember that you have to purchase travel insurance within 7 days of making an initial deposit on your trip if you want the insurance company to waive any pre-existing medical conditions. That's really important if you or a family member is being treated for a chronic ailment that causes you to cancel your cruise.
Just purchased cruise insurance through CSA and they no longer cover pre-existing conditions even if you get the insurance within 7 days of booking, which we did. I think they just recently changed that, used them earlier this year and it was covered, so check the policies before you pay for it.