Gotta be the Oracle games without a doubt.
Ocarina of time, duh. While ALttP set up the current Zelda formula, OoT was the first ever 3D Zelda and set standards not only for every Zelda game to come, but gaming in general.
OoT basically spawned combat in a 3D game, and without OoT, gaming wouldn't be what it is today. Plus it created Z-targeting, it's the game that the timeline is based off of, it has that classic Zelda story, and has one of the best and DEFINITELY most memorable soundtracks of the franchise.
The fact that it's the highest rated videogame of all time speaks for itself.