The only "persistent" villian I can think of, off-head,
is Garada-K7
(second place goes to Doublas-M2, as a "supporting actor".)
Garada and Doublas appeared together in the first two episodes
of the Mazinger-Z series...
...these two villians appear again, in the Mazinkaiser series
episode 1; and then once more: fused together with a third villian,
as a huge bio-mechabeast, in 'Kaiser series episodes 5 and 6
(in which this villian hybrid dunks the 'Kaiser into a volcano
at Mt Fuji).
Although these were destroyed pretty quick, they did give
the two Mazins a hard time.
I'm intrigued that Garada and Doublas became two of the
most popular favorite villians, all around- idolized by fans
and toy manufacturers alike.
zozo-Magengar
Villians are cool, albeit they end up dead usually after one episode. But you cna ask the Rider and Ultraman, and Godzilla fans, we buy baddies all the time! lol