Both metalcore and black metal have produced a smorgasbord of horrible bands for different reasons. Black metal's rotting core can be attributed to its ease of composition and subsequent popularity with bedroom and basement composers who can't assemble a band to save their lives (much like djent). Metalcore, on the other hand, was a trending genre, and thus suffered from a massive host of bandwagoners producing contrived tunes for a quick dollar (much like djent).
Pointing to one as holistically better than the other is a fool's errand.
However, as indicated, djent suffers from both of these dilemmas, so while I enjoy some good Animals as Leaders and Chimp Spanner, gun to my head, I'd probably have to say djent is the worst metal subgenre.