genomics - Which functional annotations could be useful?

Analyzing a genome, for a generic gene, which functional annotations (e.g from Gene Ontology) can help understanding its meaning/function or, at least, provide helpful informations?

• Annotations of that gene


• Annotations of orthologous genes (?)


• Annotations of paralogous genes (?)


• Co-expressed genes (given a biological condition) (?)


• What else?

Thank you.