Very hard to say, I might actually have to divide my vote between ALTTP and OOT
ALTTP created the current style of game play building upon LoZ, but keeping a few elements that were successful in AoL such as the magic meter, spells, magic hammer to remove obstacles, and townsfolk. While adding to that with the current three dungeon, master sword, seven dungeon, final fight setup. And created the mythology behind the game such as the three goddesses and the creation of Hyrule. In fact the entire mythology of the game and everything we know about the Triforce and Hylians in Hyrule was firmly established in ALTTP Many of the classic items like the bottles for keeping potions of fairies, the hookshot, and mirror shield. Even the traditional multiple worlds aspect, and Link venturing back and forth between them began with ALTTP. The underworld of LoZ somewhat began this but they are considered to be simply the dungeons of the game these days.
OoT formed exactly how a 3D Zelda game was to be played, in controls and items. While establishing the different tribes of Hyrule and gave an origin to Ganondorf. The rest was merely building upon the world that ALTTP established and gave more details into the origins of people and places. The game definitely took the Hyrule of ALTTP and made it into 3D adding a few more elements. However the gameplay mechanics of OoT have been repeated in every 3D Zelda ever since, especially when it comes to puzzle solving. Puzzles were rare until OoT and have now become a solid aspect of the series, now people expect to see several puzzled per dungeon. OoT also established the different "types" of dungeons that would be a pattern in each game to come after, such as the Forest, fire, water, shadow, etc dungeons. ALTTP did alittle bit of this but never made much of the elements or themes of each dungeon.
So the way I see it, ALTTP created the fundamentals that would be the foundation of a modern Zelda game. The traditional items that are expected to be seen in each game such as master sword, bottles, pendants/spiritual stones/goddess pearls, the concept that some of the monster boss fights that require a strategy, and the story aspects and mythology behind the game that are still used in all sequels today are ALTTP's contributions. While the entire 3D control scheme, puzzle solving, themed dungeons, and various tribes were inventions of OoT. So the world and the items used in it are attributed to ALTTP, while the means of using them and controlling Link in all games that came after are from OoT. I cannot make a solid decision which contributed more since every game after now requires the contributions of both or else they could not be a true Zelda game. Too many important elements would be lost.
