I have seen each episode at least once. The only exceptions are the three most recent episodes (16-8, 16-9, and 16-10), which I believe have only aired once in the U.S, and I missed those air dates.
Actually, I've seen nearly all of them multiple times, especially prior to Season 12/13. Seems like Season 12 and thereafter, I've only seen once, maybe twice, as the stories just don't seem as good as they used to be. That's not to say there are not some great episodes anymore... ("George and the Missing Puzzle Piece", "Night of the Tibble" and "Get Smart" I seemed to really enjoy)