The leader is whoever the de facto leader of Cuba is, regardless of their specific job title. Currently this is First Secretary of the Communist Party of Cuba Miguel Díaz-Canel.

If there is a complicated messy civil war, this goes by who controls the capital and governing apparatus. I intend to go by a similar standard to the late CIA World Factbook (RIP) or the Wikipedia infobox, though I don't commit to matching them if they're outdated or inaccurate.