Look, I've gone down this rabbit hole a few times, and 'best' is tricky—it's so subjective depending on whether you want them as rivals, heroes, or something darker. The classic slow-burn that gets recced a lot is 'Between the Lines' by tessera. It starts post-graduation, with Kirishima and Bakugou on different hero agencies, and the burn is glacial. It's all about missed calls, shitty texts, and Kirishima overanalyzing every single interaction for three years. Some people find the pacing frustrating, but that's the point, right? The tension comes from their careers getting in the way, not miscommunication for its own sake.
I also keep going back to 'Red Riot, Dynamight' which is a weird one because it's an AU where they're both in a support gear engineering course. The romance is secondary to the plot about building a new type of armor, and the attraction simmers under the surface for like 40 chapters. The author really gets Bakugou's voice—grumpy, hyper-competent, but weirdly soft when Kirishima is struggling with a weld. The slow burn here feels earned because they're building literal and metaphorical foundations together.
Honestly, a lot of fics tagged slow-burn in this pairing aren't. They jump from snarking to kissing in five chapters. The real gems are ones where the 'burn' is in the character development, not just the pacing. Like that one where Bakugou has to learn to apologize, genuinely, for something he said years prior, and it takes him 80k words to even understand why it matters. I'm blanking on the title, but it's out there.