Identical twins.

Here's why:

* Identical twins develop from a single fertilized egg that splits into two embryos. This means they have virtually the same DNA, making them genetically identical.

* Fraternal twins develop from two separate eggs fertilized by two different sperm. They share about 50% of their DNA, similar to any other sibling pair.

* Siblings share about 50% of their DNA on average.

* Parent and child also share about 50% of their DNA.

* Unrelated individuals share a much smaller percentage of their DNA, with variations depending on their ancestry and population.

Therefore, identical twins are the closest genetically, sharing nearly all their genetic material.