I would say Takudzwa Ngwenya. He burned Bryan Habana.
The fastest I ever saw in competition was Martin (Chariots) Offiah. Played most of his career in League (because it was professional), but started and finished in union, and had -famously - 6 tries against Bath Rugby in their one-off League game. [He also once had 10 tries in a game [v. Leeds if memory serves.] Simply ran away from people , including under heavy pressure. Find some old film on YouTube if you have not seen him in action. Mark L.
