Which actor appeared in the most episodes on any tv series?

Joseph Ruskin appeared in all five live-action series, although only in a Next Generation movie, not a television episode.

Majel Barrett appeared in three of the series, but provided the computer voice for all five, and also for the short-lived animated series.

Erick Avari appeared in at least 3 of the Television series: TNG, DS9, and Enterprise, playing a Klingon, Bajoran, and human, respectively. Some sites say he also appeared on VOY, but if true it is uncredited.