When you try to think of actors who appeared in all six seasons, a few names come to find. The core group - the Oceanic 6, plus Terry O'Quinn, Josh Holloway and Daniel Dae Kim. L. Scott Caldwell and John Terry, who played Rose and Christian. A little more surprisingly, Lillian Hurst and Andrea Gabriel, who played Carmen and Nadia.
Oh, and Chris Candella. He played a background survivor who had no lines.
Ah, if only the producers had made Chris a main character in season 3 rather than creating a new character in Pablo Poops-a-lot. The two even looked alike!