The most radiosensitive tissue is -
Radiosensitivity of important organs in decreasing order.
Bone marrow > Testes > Ovary > Kidney > Liver > Lung > Heart > GIT (Stomach > Intestine) > Thyroid > Brain & spinal cord (CNS) > Skin + Cartilage > Adult bone > Vagina > Uterus.
Brain
Bone marrow
Thyroid
Liver