The structure that would provide the best evidence that a cell is eukaryotic is the nucleus.

Here's why:

* Eukaryotic cells are defined by their membrane-bound nucleus. This compartment houses the cell's genetic material (DNA) and is essential for regulating gene expression and cell function.

* Prokaryotic cells, on the other hand, lack a nucleus. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

While other structures like mitochondria and endoplasmic reticulum are also present in eukaryotic cells, the nucleus is the defining characteristic that sets eukaryotes apart from prokaryotes.