I think the J-shape is most underrated. Even low dan players sometimes don’t know about it.
game sample: snakesss supporter vs. mrae28
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Great shape to know. The Straight J group creating a ko is a very helpful shape to know too. Rather than making a hanging connection, one can play the descent to create a ko rather than a dead shape.
Surprisingly complex for a small corner! 
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Enough complex (and common) to be a good introduction to the complexity.
Not so underrated as being one of the most well known shape in the corner.
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I learnt this shape shortly after the L-group when I was about 10k but I can see from experience in my games that many mid-SDK players don’t seem to know it.
See here https://en.1200igo.com/副本-入門題目 part 26 for a list of problems on the J-group.
In France it has been a favourite tsumego of Fan Hui and MDZ and they taught it to many of their students, under the name “pig’s snout”; I had to click your link to know what was meant by “J group”.
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