Easily A Link to the Past/Oracle Series/Link's Awakening Link (yes, they are all the same Link). This Link has been on more adventures than any other Link has, giving him an obvious edge in combat. He has also received many powerful weapons along the way that are beyond that of some of the other Link's. His health would be through the roof considering how many pieces of heart/heart containers he has managed to find throughout his four adventures, not to mention his Master Sword is the strongest of them all.
His closest contenders would be Adventure of Link, Twilight Princess and Ocarina of Time's Links. Adventure of Link no doubt has some very powerful spells, but A Link to the Past Link's Mirror Shield could bounce his more powerful spells, if many of them would even do significant damage. Twilight Princess Link has the hidden skills, but they're useless from a distance. A Link to the Past Link has the sword beam ability that would be crippling to him. If he used his Wolf Form, the Golden Sword could slaughter Twilight Princess Link. Ocarina of Time Link just doesn't have the ability; he's more power than efficient weaponry. A Link to the Past Link is smaller, therefore more agile.
Also keep in mind the timeline placements. A Link to the Past Link generally lives in a more dangerous world than most of the others - aside from Adventure of Link's Link. He lives in a world where Ganondorf/Ganon has basically reigned over Hyrule due to Ocarina of Time Link's defeat. To be able to defeat an enemy that a previous Link could not defines a more powerful fighter than the one who was defeated - in this case, Ocarina of Time Link.
In many cases, experience and better weapons will determine the strongest Link. A Link to the Past Link has the most amount of weapons, and weapons with the best upgrades. Silver Arrows are easily better than fire, ice or even light ones. His Master Sword is the most powerful of them all as well - the Golden Sword is even stronger than the Biggoron Sword. His shield is on par with Ocarina of Time Link's, if not better; it has received more upgrades than Ocarina of Time Link's did after all. Even his slingshot is better - the Hyper Slingshot is a fine substitute for the bow and arrow.
The seeds also give him an enormous advantage - the Pegasus Seeds especially. They can easily allow him to run circles around any other Link, the seeds make him even faster than Skyward Sword Link when he's sprinting. The Scent Seeds can also make a diversion - it can attract unwanted enemies, distracting the other Links. He'd sneak up behind them and deliver the ending blow right then and there.
Every Link is generally very powerful, but A Link to the Past/Oracle Series/Link's Awakening Link is easily the best. He's the most agile, experienced, and has the best weapons, giving him far too many advantages for any other Link to even have a chance of winning against him.