Okay, so you're asking about the really heavy stuff for ShinBaku, the ones that stick with you. I've read a ton, and honestly, a lot of them miss the mark—either it's all smutty rivals-to-lovers with zero buildup, or they make Shinso way too soft, which just feels wrong for him. The gems are the ones that understand both characters are fundamentally lonely, just in different directions. Bakugo's loud about it, Shinso's quiet. The fics that get that dynamic right, where their damage actually complements instead of just colliding, are rare.
'Phantom Pains' by ghostwords on AO3 is probably the best I've ever read. It's a postwar fic where Bakugo has chronic pain from his injuries and Shinso's insomnia is debilitating. They don't even start off liking each other; they're just two insomniacs stuck in the same physical therapy group. The emotional depth comes from how they communicate—or fail to. It's all in the quiet moments, Shinso using his quirk to pull Bakugo out of night terrors, Bakugo forcing him to eat when he's too tired to remember. It feels earned.
There's another one, 'Echo Location,' that plays with the idea of voice. Shinso is losing his quirk due to a side effect of a drug used during the war, and Bakugo, whose entire identity is tied to his loudness, has to learn to communicate without yelling to help him. It's messy. They hurt each other sometimes, say the wrong thing. The author doesn't shy away from that. The comfort isn't fluffy; it's two jagged pieces finding a fit that's not perfect but works. I keep coming back to that one when I want something that feels real, not just wish-fulfillment.
A lot of people rec 'Reverse Engineering' for the clever quirk analysis, but for pure emotional gut-punch, I'd say 'Phantom Pains' is the peak. It ruined me for a week. The ending isn't even traditionally happy, just... peaceful. Which, for them, might be better.