So, romantic tension between Kara and Lena is basically the entire Supercorp fandom's lifeblood at this point. There's a ton of fics that stretch that 'will they, won't they' feeling across whole universes.
My absolute favorite for slow, aching tension has to be 'The Art of Letting Go' by somuchtoremember. It's an AU where they're rival art curators, and the push-pull is just exquisite. Every interaction is charged with this competitive respect that slowly melts into something else. The dialogue snaps, and you can practically see the stolen glances. It’s not even about superpowers or Luthor legacies; it's all in the quiet moments and professional rivalry turning into something more.
Another classic is 'City of Glass' by seraphinasnape, which is a canon-divergence where Lena finds out Kara's identity much earlier. The tension comes from the betrayal and the forced proximity as they have to work together. The emotional whiplash of anger, longing, and reluctant trust is so well done. You’re constantly on the edge, waiting for that one moment of vulnerability to break the dam.
Honestly, some of the best tension I've found isn't even in the million-word epics. Short, focused one-shots like 'Proximity Alarms' by tectonic, where a Kryptonian soulmate bond manifests as a constant, low-grade hum that gets louder the closer they are, just kills me. It’s all internal conflict and stolen moments.