George Formby (Jr) was so popular in the 1930s and 40s that it became fashionable later to belittle him, as he was seen as being old-fashioned and representing a way of life that people had moved on from. But looking at him objectively now, IMO he was indeed one of the all time great performers.

As for the Singing Postman (Allan Smethurst), he was good too, but I wouldn't say he was better than GF. They're both very different artists. AS certainly wasn't as successful as GF (but then not many artists have ever been - he was phenomenally so). But then again success and quality are not always in direct proportion to one another.