Though I still reckon Young Link is the most powerful Link of them all! Not even ALttP could beat him.
Let us look at this logically.
Young Link from Majora's mask is immensely powerful yes, but he doesn't beat ALttP Link.
ALttP Link has more sword power with his Golden Master sword than with YL's Fairy Sword.
YL gets the Kokiri Sword, which is standard, but then doubles it to the amount of the Master Sword with the Gilded Sword, and then the Great Fairy Sword is twice the damage of the Gilded Sword and by comparison, the regular master sword.
ALttP Link has the Master sword, which he forges to the Silver Master Sword, which has twice the power of the Master Sword(And the same power as the Fairy Sword), and then forges it again to have twice the power. That is a 4-8 ratio. It barely is a fight.
Young Link also has better resistance to attacks, he takes 1/2 damage than most other Links.
However, ALttP Link has even better resistance, he takes 1/4 damage to attacks.
Young Link also has access to Magic Arrows, which does twice the amount of damage, and the light arrows,, which does 4 times the amount of damage.
ALttP gets the upgrade to his bow in the form of Silver Arrows. These arrows do twice the damage than a normal arrow.
In this one, YL is superior, no question about it. However, YL is tied to his magic meter for all of these, and Light arrows cost a ton of magic.Furthermore, YL will be doing the same amount of Damage to ALttP Link as he does to him with their bows due to ALttP Link's better tunic.
Hookshot remains the same for each Link. So that is equal.
Bombs are also equal, unless you count the Goron Bombs, of which those would be useless in a real fight.
Now, I know what you are going to say.
The transformation masks and masks in general.
Lets go down these, shall we?
YL has the Deku Mask:
Well, this doesn't really help YL too much.... Deku Link is very bad offensively, and the only thing Deku Link could really do is run away. Of which ALttP Link can easily use an arrows, or Fire Rod.
YL has the Goron Mask:
This one has some valid point. Darmani can roll around, and deals heavy blows.
ALttP Link has bombs as well however, that can stop Goron Link from doing his offensive roll, and Goron Rolling is also coincides with the Magic Meter. Furthermore, ALttP Link has Ether, of which can easily freeze a Goron. ALttP Link also has the magic Hammer, of which it deals similar blows with similar ways to match Goron Link's.
YL has the Zora Mask:
On Land, I don't see much use for this outside of the boomberangs he has. Of which, ALttP Link has a better boomerang, because it has better range.
On sea, Mikau is superior. He can breath underwater forever, and has more maneuverability. However who says this is going to be a water fight? ALttP Link could easily leave the water, and just wait until YL is man enough to fight on land. All while sniping Mikau with arrows
YL has the Fierce Diety Mask:
This mask has very limited uses, but even if YL were able to use it, it would not help.
The Fierce Deity Sword does as much as the Golden Master Sword,
Can shoot beams out of it too, but it is tied to magic meter.
FD Link also doesn't have any versatility in items that ALttP Link has, and ALttP can always use the Cane of Byrna, become invicible, and wait things out while simultaneously doing damage to YL.
The Bunny mask give YL a movement speed edge, but ALttP Link has the Pegasus boots and can run faster while doing damage, albeit he can only run in a straight line.
So yeah. It would be a close fight, but ALttP Link is just simply better.
Also, ALttP has the full Triforce.