I too would like to see someone more verse in the nuances of the various BSDs answer this question, but I'll give it a go.

DragonflyBSD is:

- focused only on x64 architecture

- has an extremely small but exceptionally talented team of developers

- has it's own unique filesystem called Hammer (and work is being down on Hammer2 which is a complete rewrite)

- Network performance is particularly good with Dragonfly [1]

[1] https://leaf.dragonflybsd.org/~sephe/perf_cmp.pdf

Edit: typo