this is a very broad question. a good introduction would be;

Cancer, Causes and Prevention. Swedish Cancer Committee 1992 . taylor and Francis.

detailed quantitative data can be found in the WHO/GBD study papers, some of which are on my RG site.

the question of "importance' is itself critical: do you want total deaths preventable, total disability preventable, controllability, strength of evidence, treatability, economics as a criterion.

to mention a few: smoking,alcohol, ionising radiation, UV radiation, non ionising radiation, PAH, benzene,,diet, asbestos, other chemicals