Male most common:
Prostate
Lung
Colorectal (bowel)
Bladder
Cutaneous melanoma (skin)
Female:
Breast
Lung
Colorectal
Endometrial (Uterus)
Non-Hodgkin lymphoma (spleen, lymph nodes or tonsils)
This excludes many childhood ones, but for adults you also have leukaemia (blood or bone), pancreatic (gland), Esophageal (throat), head and neck (lips, mouth nose etc) and ovarian cancers which are also relatively common.
Most PMIs exclude cancer from their general indemnities as it is contentious to define whether the condition was existent before the policy was taken out (inherited or genetic) or self-inflicted (carcinogens etc).