Evolutionary relationships?

Question

what sort of features of similarity are the most useful for discovering evolutionary relationships?

Answer 1

The gold standard today is nucleotide sequences of ribosomal RNA genes, or of mitochondrial DNA

Answer 2

Suites of anatomical characters and genetic molecular analyses.  Which particular characters are useful will vary among different groups of organisms.  It's not a particular sort.  Rather, it's any sort that's available and informative.

Over-reliance upon single details can result in confusion from convergent evolution; similar structures arising independently by different routes.