To be technical the Rangers have never had a "leader", though Hiro will always remain the founder and spiritual heart of the Rangers as a group. So the roster was never designed to distinguish any one Ranger above another. Every character is free to be their own man/woman.
That being said, since Duke has left, myself and Zay have taken over as Mods for the Rangers. And by association, our characters [Lexi for my part, and Zaylos for Zay's part] are the interim "leaders" who would deal with most IC Ranger business like important missions, story arcs, or negotiating with other factions.