I find the question almost impossible to nail down to just genres, because popularity swings wildly based on fandom mood and the source material's own gaps. Romance and smut will always top the charts in terms of raw numbers—people love exploring those 'what if' scenarios between characters, filling in moments the canon glossed over.
That said, some of the most talked-about, feverish discussion I see tends to cluster around specific subgenres within bigger stories. Hurt/comfort, for instance, especially in action or fantasy settings. Take something like 'The Last of Us' or 'Supernatural'; readers crave those intense, emotional aftermath stories where one character patches up the other, physically or emotionally. It creates a vulnerability the original plot might rush past.
Then there's the whole phenomenon of Alternate Universe—coffee shop AUs, superhero AUs, fantasy role reversals. Their popularity isn't about the 'coffee shop' per se, but about transplanting beloved dynamics into a safe, often lower-stakes setting where the relationship can be the entire focus. It strips away plot armor and lets the character chemistry shine in a new context. For me, that's where the real magic often happens, far from the epic battles.
I'd also toss in the 'fix-it' fic for tragic or unsatisfying endings. It's less a formal genre and more a powerful reader impulse. After a finale that leaves everyone devastated, the fandom collectively writes a happier path. The drive to heal that canon-provided hurt generates an insane amount of passionate, heartfelt work that resonates because it comes from a shared sense of loss.