I'd say the series as a whole is underrated. Ones that I realize most people don't seem to talk about are most of Cotterpin's episodes, Wembley's Flight, Fraggle Wars, and The Terrible Tunnel. That last one is on my list because in the Season 1 set, they say Marooned was the first life threatening case, but it's obvious to anyone who knows the order, and saw the Terrible Tunnel it's not quite true.