Close family member. It's best if you designate someone to make decisions for you. A medical power of attorney can follow your wishes, otherwise your closest family member may be required to make decisions on your behalf.
Next of kin... Your next of kin would make these decisions for you. You can also fill out a living will document and/or a medical proxy and name the person that you would want to make your decisions especially if this person is different from your next of kin. Important to also talk to this person ahead of time and let them know what your wishes would be so they won't base decisions on what they want for you.
Your next of kin. Legally your spouse, next children.
State laws dictate. Most stated have laws dictating who becomes you surrogate decision maker if you are not able. Usually it is your spouse then kids then parents. What is often recommeded it a health care power of attorney in which you can designate anyone you choose to make decsions for you.