Final answer:
Mitochondrial genes are the most useful for barcoding non-plant eukaryotes because they have a high rate of evolution, making them ideal for distinguishing between species.
Step-by-step explanation:
The most useful genes for barcoding non-plant eukaryotes are mitochondrial genes. DNA barcoding involves sequencing a specific section of genetic code, and mitochondrial genes have a high rate of evolution, making them ideal for distinguishing between species. Other genes, such as nuclear genes and ribosomal genes, are also used for barcoding, but mitochondrial genes are typically the most informative.