Nc4 clearly as the computer says. You can take on a4 next move and threaten mate on a1.
Mate Threat vs. Queen Attack

As the saying goes, one win per game is all you’ll ever need. That said, if you find a good move, find a better one—as it could be the difference between winning and drawing or losing.
At 15.d4 I saw to possibilities, 15...Nc4 threatening the queen, and 15...Qxa4 threatening mate. Since White's Queen was somewhat inactive and holding a defensive position, I decided 15...Qxa4 was better. Thoughts?