The hardest level that has been verified legit is Sonic Wave by Cyclic. However, this level was taken down by the publisher, and Riot is trying to re upload it. Since Sonic Wave is no longer on the servers, Bloodbath by Riot is the hardest level that has been verified legit. If I hadn't specified "legit", the hardest levels would obviously be the impossible and "silent" levels which the community has, annoyingly, started to make.
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geometry dash l b612 l retrica
There are many hard levels in Geometry Dash. However, the hardest level is Silent Club by Play 1107696. Although it was listed in the “ Impossible Category”, but some people has completed it with Start Position ( Zobros completed with Start Position from 52% to 100% ). It also started a trend of Silent levels like Silent Circles, Silent Clubstep etc. It features Straight Flying, 4-spike jumping, and spam orbs. Once it was rated Demon, but Cyclic found out a hard-to-find secret way.