Tathagatagarbha, which he taught only to a select few students. These teachings were hidden, some say in a secret realm inhabited by serpents (nagas), until the time came that Buddhists were ready to receive them
This creates an obvious hierarchy in the teachings, with the earliest recorded ones assuming a foundational status while Prajnaparamita and Tathagatagarbha sutras are recognized as "higher" teachings.
But here's the thing: the historical Buddha never recited the Prajnaparamita and Tathagatagarbha sutras. It's a fact; modern scholarship supports this. We can call these sutras apocryphal, if we want; but the fact remains the same--the Buddha never uttered them.
And that's okay.
As a Mahayana Buddhist, I have no problem accepting the Heart or Lotus or Diamond sutras as being authentic scriptures, even if the Buddha himself never uttered them. And no, I'm not exercising some fancy form of Orwellian doublethink.
Just because the Buddha didn't recite these sutras, doesn't mean a Buddha didn't. Zen, in particular, recognizes that we can all reach Enlightenment in this very lifetime. Which means that there have been many awakened beings since the historical Buddha first expounded the Dharma 2,500 years ago.
Chinul, Dahui, Mazu, they were all awakened masters, and I recognize all of their teachings. So I personally don't feel the need to pretend that the historical Buddha Shakyamuni recited Mahayana sutras (which he didn't), in order to believe that they are legitimate expressions of the Dharma. A Soto Zen Buddhist has no problem accepting Dogen's teachings as the word of a Buddha, the same goes for a Gelupga lama with regard to Tsongkhapa.
With that said, I am always amazed when I hear well educated Buddhists--usually trained in traditional Asian settings--claim that they have the highest Dharma, that Tathagatagarbha or Shentong or Hua-yen are somehow higher teachings than those found in the Pali Canon, or even Prajnaparamita literature for that matter. From my Western perspective, this ranking is completely unnecessary. Buddhist scriptures do not need to fit together like jigsaw puzzle pieces because, I think, their goal is not to create a philosophical ladder, with Pali Buddhism (what some might call Hinayana, a term I prefer not to use) at the first rung and Tathagatagarbha/Buddha nature doctrine at the top. In large part because they're not all coming from the same source--the Buddha did not utter all of them.
Which, again, is fine. Just because the Buddha didn't recite the Lankavatara Sutra in no way invalidates it as a Buddhist text.
This ranking doctrines derives from the need to integrate Buddhist teachings--to make them consistent with one another--because it is predicated upon the idea that the Buddha recited the sutras, which is simply historically inaccurate.
This ranking tendency, I feel, reflects how people can lose sight of the purpose of the Dharma. The Buddha was not interested in philosophy; he was concerned with one thing only--ending suffering, i.e., waking people up. And so his teachings are upaya, or skillful means, and should not be misconstrued as statements about reality as much as provisional teachings designed to help us wake up. The moment we rank them, we reify them, which defeats the entire purpose of the teachings in the first place.
Theoretically, someone could wake up using the Eightfold Path, koans, Tantra, vipassana, or any one of the multitude of Buddhist techniques designed for that purpose. They're all effective and valid; they don't need to compete with one another for authenticity, because the moment we fall into that trap, we're attached to form, word, and letters.
And we need to let that go; let it all go, in fact.
The Buddhadharma is a magnificent cornucopia of upaya, consisting of flavors to meet everyone's spiritual palate. It need not be competitive, hierarchical, or mythologically naive about its origins.
The Dharma is ever-present and all-inclusive, and so the last thing it should leave out is an aspect of itself.
Photo borrowed from Creative Commons flickr user: godutchbaby.