What is the Hardest Trick you can do? 
With a bit of experience under your belt, none of those tricks are very hard to do.
The hardest trick is the one you are currently learning/working on.
Except Walk the Dog with a nice yoyo on a rough surface. That’s very hard to do!
Superman is my peak, I’m not even going to front. The underpass after the Magic Drop took me about a year to learn a sufficient way to do it. Spirit Bomb is also hard.
I can also “add” other tricks to the Poll, suggest them as you will.
Aces High took me a few days to learn. Swinging the yoyo through my thumb and pointer finger is challenging because my hands are so small.
Can do cold fusion about 50%, working on hydrogen bomb
The hardest trick I can do?
Staying married, paying the mortgage, and raising two kids without going insane.
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(That’s an old guy joke. You’ll get it some day.).