Probably either the Double or Nothing or 1.5 Mount. I was weird doing tricks at first on the side mount with a breakaway.
And whut. took a while for me to get down and now iccan do it in my sleep
Eli hops took me a good few months of giving up and coming back to learn.
My Boing-E-Boing is still not reliable. Curse you Boing-E-Boing. 
Boing-E-Boing took me a while too. Probably a month off and on. I had to watch Miguel Correa’s tutorial to finally hit it consistently. And, I had to stop practicing the boomerang cause of string burns – I keep forgetting to go back and practice it.
Plastic whip. After trying it off and on for a couple of months, I finally had a breakthrough yesterday, and can hit a 2-3 in a row without missing.
Brent stole. I had three Or four days where I could get it 100% of he and then I just couldn’t do them anymore
1.) spirit bomb
2) the jump rope slack trick
3.) rancid milk
Seasick
Grandma Kimmitt sandwich. I’m still working on it…
berral rolls, I learned it from memory (seen it done twice) on a fixed axle yoyo. It took from around 8 years old to around 12ish years old. could have had it down faster if I would have learned all the tricks that I needed to know to first. Since I got bearing yoyos I would have to say either dice through suicide 5A trick, or sea sick.
And that is if we are excluding 2A. Which from what people have said it looks like we are.
Seasick.
I don’t think I know how to do walk the dog anymore =(
Spirit Bomb. Still couldn’t do it. :-[
Boingy boing… Still not consistent but i can get 5-15 boings in most of the time
1.5 mount, Boingy Boing, and Spirit Bomb took me ages to learn.
Boing E Boing for sure… but now it’s my favorite
Winding the yoyo. I had to watch the tutorial five times.
Ninja Vanish. i still can only do it 2 out of 10 times! 
Magic Drop, It took about a year and a half since I started practicing on it till I figured it out