Send a ship and I’ll tell you who:
Gives nose/forehead kisses: Honestly ?? Neither of them is really the physically affectionate type? I could see both of them doing it in certain circumstances, but
Gets jealous the most: Haha, both of them, but Shinichi is a really possessive bitch.
Picks the other up from the bar when they’re too drunk to drive: Shinichi routinely drinks with the police officers / detectives after a case ( when he gets old enough ), so Kaito tends to pick him up.
Takes care of on sick days: Both of them are far used to ( not ) taking care of themselves when they’re sick, so I feel like, at first, the two of them would probably be like “oK AY YOU’R E SI CK I DK WH AT YOU WAN TT O DO HER E H AVE LI K E S OUP AN DM ED I CIN E NO ST AY IN BE D ST O PW OR K ING!”
Drags the other person out into the water on beach day: Both of them like going into the water, but Kaito would run out of the ocean as fast as possible if he felt something brush against his leg, so Shinichi would be forced to drag his ass back in.
Gives unprompted massages:
Drives/rides shotgun: Kaito only let Shinichi drive once. Once. And he vowed never again. ( AKA, Shinichi picked up his mother’s driving skills, and Kaito would like to live, thanks. )
Brings the other lunch at work: Neither, probably, because they’re both workaholics and would look at the time and go, “…Oops, guess I missed lunch. Again.” Although, I could see Kaito doing it a few times.
Has the better parental relationship: Hahahahahahaha, Shinichi.
Tries to start role-playing in bed: Kaito just jumps into another voice, and Shinichi smACKS him until he stops.
Embarrassingly drunk dancer: Shinichi.
Still cries watching Titanic: Neither?
Firmly believes in couples costumes: Kaito doesn’t actually “believe” in them, but he’d force Shinichi into one just to see Shinichi grumble and glare.
Breaks the expensive gift rule during Christmas: Kaito. Probably after one of his trips to France.
Makes the other eat breakfast: Both of them are notoriously bad at eating breakfast if they’re busy, so they both of them tend to remind each other. However, if neither of them are particularly busy, then no one needs to make the other eat breakfast.
Brings up having kids: In the far, distant future, probably Kaito, but I feel like Shinichi would just ACCIDENTALLY slip that into a conversation with his lack of conversation finesse.