Super Mario 64 is important for so many reasons. It introduced Mario into a world of 3D, at the same time legitimizing the 3D platformer, a genre other developers had struggled with. For the first time, Mario could move around a fully 3D world, which enabled Nintendo to create more expansive environments than ever before. Though the camera was often spotty, Mario himself controlled like a little hopping angel, allowing for level design that challenged players without ever becoming too frustrating.
And Super Mario 64 set the stage with large levels that were segmented into smaller missions. All that combined with the huge amount of secrets scattered around Peach's castle gave Super Mario 64 a ton of replay value, and its design is so solid that it's still incredibly fun to play today.