I'm probably going to be the weird one here, but I don't always go for the big confession scenes everyone recommends. Sometimes the small, quiet moments hit harder. There's this one fic, I can't even remember the title now, where it's just Saiki passively listening to Teruhashi's internal monologue during some school festival setup. She's not even trying to be 'perfect', she's just mentally complaining about how heavy the decorations are and wondering if she can sneak a bite of takoyaki without anyone seeing. Saiki overhears it all and, without a word, just levitates the box she's struggling with. He never acknowledges it, she never finds out, and the fic just moves on. It's such a non-moment, but it's so perfectly them—his begrudging, unseen care meeting her hidden, mundane humanity. That stuff sticks with me longer than any grand romantic gesture.
On the other end of the spectrum, there's a popular trope I actually avoid: the 'Saiki finally admits his feelings' climaxes that make him too soft too fast. It feels out of character. The better versions are the ones where his 'confession' is an action so quintessentially Saiki it loops back to being romantic. Like a fic where Teruhashi is sick, and he uses his telepathy to hear she's worried about missing a test, so he psychically copies the notes onto her desk—but arranges them in a mildly irritating, illogical order just so she'll have something to grumble about and forget her fever. The romance is in the precision of the annoyance.