In almost every sitcom or any other show, at least 1 actor would have to appear in all the shows, however, I couldn't find the answer to this one. I like to know what actor appear in all 255 shows?! Could it be Out of Practice's Henry Winkler A.K.A. The Fonz, along with Tom Bosley & Marion Ross?!

I'm pretty sure Henry Winkler missed a few shows during the first 2 seasons. He wasn't a regular yet during season 1 and was only 5th billed during season 2. My guess would be Tom Bosley and/or Marion Ross but I'm not sure.

Henry Winkler and Tom Bosley

he was in every episode i do think henry winkler missed a couple early shows im not quite sure about marion ross

Winkler didn't miss any episodes. There was at least one, possibly two, episodes that Ross didn't appear in. I made note of anyone who has missed episodes, and Winkler and Bosley are the only ones who haven't.

Winkler didn't miss any episodes. There was at least one, possibly two, episodes that Ross didn't appear in. I made note of anyone who has missed episodes, and Winkler and Bosley are the only ones who haven't.
Marion Ross didn't appear in "If You Knew Rosa".