I'd say this can be interpreted in 2 ways - either what are greatest RISKS that may lead to disaster, or what are the greatest ACTUAL disasters which I'd define as leading to greatest losss of life and / or environmental impact.
I's say the greatest risks are through fire / explosion, collision and water ingress (ultimately leading to sinking)- each of these may have a number of causes, incompetance, lack of training, not following SSOW / procedures etc..
If we're talking actual loss of life then war losses e.g MV Wilhem Gustav, Yamanto, HMS Hood and others rank highly, but there are large losses on civilain vessels too - Doria Paz, SS Kianga, RMS Titanic etc....mainly through the risk factors above.
On envirnotemtnal disasters there are the Gulf of Mexic oil spill, Torrey Cannyon, Exxon Vladez, Ivory Coast toxic waste etc..
Is this a case of TV doing an over simplified over-hyped shallow, poorly reasearched piece playing to "popular views" or using "good footage"? No, TV producers wouldn't do that would they?