Aww, I was gonna put Sarek in there, but only held back because he wasn't a major TNG character. But I guess two appearances count for a lot with a legacy character.
Of any recurring
characters of TNG, which ones appeared in any other Trek show?
Lursa & B'Etor
Taking your definition of recurring in TNG to mean appearing more than once but not a main char, otherwise I could expand that list to a lotta different people.