It depends on your definition of famous and when you started watching. I saw Roots and was pleased to see LeVar Burton on TNG (before realizing what a boring character Geordi was), but if we're talking about TNG actors, I'd have to say Patrick Stewart. I saw him many years before, in I, Claudius, Dune, and Lady Jane. Of course he was in other movies, as well as his stage performances.
For TOS... I have no idea which of them was more famous, as I'd never heard of any of them before seeing Star Trek.
DS9... I'd heard of Rene Auberjonois (having seen a few episodes of Benson).
Voyager: The only one of them I'd heard of before was Kate Mulgrew, and only in connection with that silly "Mrs. Columbo" series.
Enterprise... well, of course I'd heard of Scott Bakula - loved him in Quantum Leap. Never heard of the others, though.
I've since seen quite a number of the various Trek actors in other roles. If you want to catch a lot of the TOS actors and guest stars in older shows, just watch the first half-dozen seasons of Bonanza - there's even one in which both Ricardo Montalban and Madalyn Rhue guest star as husband and wife.