That's a surprisingly tricky question because popularity shifts with adaptations and fan moods. Currently, Bruce Wayne and Clark Kent absolutely dominate spaces like Tumblr and AO3. The dynamic of 'world's greatest detective' with the 'big blue boy scout' offers endless tension—ideological clashes, mutual respect, a foundational trust that can be bent into romance. It’s the ultimate power couple fantasy. I see less fanart for Bruce and Selina Kyle now than I did a decade ago, though 'Batman Returns' probably cemented that for an older generation.
What’s interesting is how the BatCat ship thrives on a different fuel: it’s canon-adjacent, a will-they-won’t-they with real weight in the comics, so fanworks often explore the melancholy of their missed connections. Bruce/Dick Grayson has a massive, fiercely protective following, but it’s often relegated to more niche circles due to the obvious problematic elements, though fans argue it’s about the evolution from mentorship to equals. For pure, unfiltered id, the Joker pairing remains shockingly resilient—it’s all about obsession and dark mirroring, less romance and more destructive psychoanalysis.
Honestly, metrics from Archive of Our Own tag counts or Reddit polls will tell you BatSuperman is the statistical winner, especially after movies like 'Batman v Superman' gave fans so much material. But walk into a convention and you’ll see just as much BatCat merch. It really depends on which corner of the fandom you’re in.