I found four cool riddles on Youtube, which are quite hard. They were all made by TedEd, who make lots of educational videos. Here they are:
The Giant Cat Riddle:
The Wizard Standoff Riddle:
The Seven Planets Riddle:
The Buried Treasure Riddle:
I hope you enjoy there riddles! If you want to see more of these riddles, you can find them on Youtube or on the TedEd website!