Mitochondrial
Mitochondria are tiny compartments that are present in the cells that make up most of the living things with which you are probably familiar (such as plants and animals). They use the oxygen you breathe to burn the food you eat, giving you energy. In a sense, they are foreigners living in your cells, because they are descended from bacteria that took up residence in the cell lineage from which we are descended, more than a billion years ago. Because they come from an independent cell line, they have their own DNA. Unlike most DNA, which is inherited from both parents, this DNA is only inherited from the mother, and this makes it useful in studies of evolutionary relationships. Since you inherit it from only one parent, you only have one copy of the genes it encodes (again, unlike most of your genes, for which each parent contributes a copy), which means a scientist trying to read one of these genes isn't stuck trying to tease apart two overlapping copies. Also, the scientist knows the entire DNA strand has the same evolutionary history, because different copies never come in contact with each other to exchange genes. Finally, this DNA tends to evolve quite rapidly, in part because a new variant only needs to spread one copy to all of the females, instead of two copies to everyone; this means mitochondrial DNA lets you distinguish between family lines that may not have evolved differences at other genes.