Day 21 is ‘favourite recurring character’.
There are two contenders, both from Deep Space Nine. Dukat is a duplicitous, wily schemer, who wants to regain his place of power over the Bajorans. He irrationally believes they owe him, for apparently ‘looking after’ them during the Cardassian Occupation. His duels with Captain Sisko and Major Kira make for some compelling viewing, and he’s one of the most interesting villains Star Trek has ever had.
Damar is a great example of a character who starts out quite insignificant, but in the end, holds the key to preserving an entire way of life. He starts out as a sidekick to Dukat, but events push him into a much more important role, and further events lead to him starting a rebellion.
Between the two, I’m going to pick Damar, as he is a fantastic representation of the evolution of a character, from nobody, to despot, to hero.