There used to be an old wives tale that "cells" replaced themselves every 7 years....I understand it's a daily process not something that happens on a timed schedule. I'll say cells.

Skin

I will go with skin also.

Bone marrow

Skin replaces itself much more frequently that every 10 years.

I'll guess the brain.

Taste buds, until a certain age is reached, then they are not replaced. Last to go are the ones for sweet (I'm on my way to the freezer for some ice cream!).

Bones?

Or maybe blood cells?

Rusty

Eyes

cartiledge