World Record AD Zed
This is not something that would be practical to do in an actual game. It would require a group of people on both teams to work together, and it would take a long time to accomplish.
So you can build whatever items you like on Zed in order to maximize his AD, but in order to truly reach the most AD possible you have to use the passive portion of his ultimate.
This means that you have to kill an enemy champion with a stupid amount of AD.
So you have to have Zed kill the "World Record AD Jhin"
To get that Jhin and his team will need 3 Infernal Drakes and will need to have the Elder Dragon buff active at the time.
I'm actually between two item builds on Jhin.
or

Both of these would give him 100% critical strike chance. The first one would give him more AD before applying his passive, but the second will still give a lot of AD and would make the most use of his passive.
He will also need attack speed Marks and Quints.
For Masteries 5 Fury 1 Fresh Blood 5 Natural Talent 1 Bounty Hunter 5 Battering Blows 1 Deathfire Touch
And for the last 12 points, ... you can really do whatever you want.
For Jhin's allies, Lulu, Nunu, and Nidalee can buff his attack speed And an equally fed AP Janna can increase his AD even further
So, all of this together can make for a monster Jhin, but this isn't about Jhin. This is about Zed
And this of course all depends on if Zed's ultimate actually works the way i think. My understanding of his ultimate calculates the AD of the target AT THE TIME OF DEATH. So as long as he kills Jhin when he is fully buffed, Zed should gain 20% of that AD permanently.
The most difficult thing about getting Zed to actually kill Jhin with his ultimate would actually be Janna's shield. My theory is that you will have to have Zed kill Jhin with max AD without Janna's shield in order to get that AD boost first, then try it again afterward with the shield. Jhin will also probably have to be nearly dead before the shield is applied.
From what I was able to calculate, before the dragon and allied buffs, Jhin should be around 1200 AD. After these buffs, he may be close to 2000.
20% of 2000 is a bonus 400 AD
So again, this would be impractical to accomplish in an actual game, but I think it could be fun to pull off.
You would have a Jhin and a Zed who could LITERALLY one-shot pretty much anyone.