Right now, almost any 3a trick…
Most lacerations, like iawasa tower, 2.0 hook, and things like that are killing me too.
A good while ago the zipper was literally impossible for me… I tried it the other day and you would think its the only thing I had practiced since I first tried it nearly two years ago
For me it was White Budha back when I was using a Duncan Butterfly. My friend was using a DV888 and he showed me the trick. The whole time he was doing it I was thinking in my head: I will never be able to do that, I will never be able to do that! And now every day the trick looks easier and easier (but still really hard).
Like a lot of people here, I would have to say Boingy Boing. I mean, I used to try to do that trick all day. I just never got the motion down, so I just stopped trying to do that trick. But when I was more advanced, maybe 4 or so months later, I tried it and just did it the first time. 
Boingy Boing is probabbly everyones first step.
After 4 Months of yoyoing start to Finsih you’ll get it
Its just like a Boomerang motion.
I am currently learning Protrusion which is Under boing-boing + Boingy Boing + Transitions .
Eiffel Tower laceration took me a while to get going. So did 2.0+ hooks.
PROTRUSION!!!
It used to be brent stole
boingy boing
One day for me boing e boing just clicked so my advice to everyone trying that is to just start realllly slow and try and get a “feel” for the motion. just start off very slow. the day i decided to do that was the day it clicked. in all seriousness I CANNOT do the last hop of spirit bomb. I learned the trick Akira yesterday in 25 minutes but I can’t do that last hop in spirit bomb for the life of me. on another note Akira is a great intermediate - advanced trick. LOVE it.
Boingy Boing and Eli Hops just clicked one day same with landing a hook. Ninja Vanish took me awhile too. Also, I finally understood the last part of black hops after watching the yyf pgm2 vid.
Seasick
Still can’t do it properly
you have to make your own rhythm with it also my yoyo goes completely straight in midair than shoots back
When i knew nothing what soever about yo-yoing, i thought that the eifel tower was hard. Know matrix is impossible for me. as is rewind.
^this
I can’t for the life of me do a good seasick.
I’ve been getting the hang of it but still can’t do it that smoothly.
I spent about 2 hours of total practicing and BAM it happened.
I then instructed myself on over-under.
Hardest trick for me: Anything from Takahiro Lizuka
Hardest tricks for me for a while were iron and jade whips. I can do whips all day with offstring but just couldn’t get the hang of 1a whips till about a year ago when it finally clicked with me.
OMG Yes!!! Whips where my weak link when I was learning.
I can do practically everything but boingy boingy and spirit bomb… everything else (not including seasick) was fairly easy.