It's a very good exercise. One of my favorites, but I wouldn't say it's the only one worth doing or the hardest.
Harder:
Front lever raises
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Human flag
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Standing ab wheel roll-outs
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Also good:
Decline sit-ups (weight held behind head)
The nice thing about the decline sit-ups is that it doesn't tax your shoulder girdle muscles like the other exercises and it won't affect your other upper body movements.