-James Sloyan (Jarok/Mora)
-John Vickery (Rusot)
John Vickery cannot be on your list. By your own definition, the actor (or actress) can only have played a single character in up to a three-episode arc. John planed three characters on three series - a Betazed in TNG, a Cardassian in DS9, and a Klingon in the Abomination.
Sloyan had even more characters, including the Bajoran scientist who Odo tried to copy. He was also a Romulan on TNG, something on Voyager, and an adult Alexander on TNG.
For actual one-time guests, I'd like to see David Ogden Stiers - Timicin - interviewed.