Going off what I said before… While I think Tsukishima definitely developed feelings first, Yamaguchi is probably the one who’d confess first.
Yamaguchi wouldn’t have even realized he liked Tsukki that way until high school, probably halfway through, and would think that he was being irrational or that he was interpretting his own feelings wrong or something. By the time Yamaguchi is a second year, though, he’d have more confidence than we know he has now, so it may actually make sense that he’d just go for it. It wouldn’t be fair to keep this from his best friend, after all, right?
So, he tells Tsukishima about this relatively-recent development, and Tsukishima is the one who panics. He doesn’t know what to do because he’d been pushing this stuff down for so long, he was almost certain it’d disappeared. But, obviously, it hadn’t. It probably creeped its way back over the years since then, especially once Yamaguchi seemed to grow a spine, and now that his best friend was telling him outright that he reciprocated this feeling, Tsukishima wouldn’t know what to do. It’d be like if you sarcastically tell someone to buy you something and then they whip out their wallet and get it for you – you have no idea how to react because you didn’t prepare for this to ever happen. Yes, you WANTED it, but as far as you were concerned, it wasn’t in the cards, so the response to it tends to come out rude and dishonest, like telling them to take it back or that you were just kidding or that they were crazy.
Saying something like that to someone who’d just confessed to you would be really hurtful, though, and Yamaguchi would end up just figuring that he was dumb for trying to act on his affections like that, ask that they stay friends, and Tsukishima would agree (probably without any verbal reply). It’d keep eating Tsukki up inside, though, and Yamaguchi would just work his regret out but keep bringing it up in conversation like it was some big joke that he’d never meant for Tsukishima to take seriously in the first place, and finally Tsukishima just snaps and admits that he’s had this HUGE crush on Tadashi for YEARS and he never knew what to do about it, so he just kept it to himself and didn’t understand what to do when Yamaguchi confessed, which is why he said what he did when it happened.
Yamaguchi would just laugh at him, because wow, Tsukki, you’re a dork, and tell him that he’s grateful he said so and then Yamaguchi accepts his confession and Tsukishima gets all miffed because Yams confessed first, but they end up dating and everyone else is all confused that they weren’t already dating before then, and they live happily ever after~~