Makoto: His lower back tends to be his most ticklish part. The moment someone starts tickling him he'd probably scream and then just start laughing, he'd be very panicky about it since it would have scared him at first.
Haruka: The stomach is the most ticklish part of his body. The moment someone starts an attack he'd completely lose his composure and break down laughing.
Rin: His neck is the most sensitive part of his body. Anytime someone touches his neck the wrong way, would have him shooting up his shoulders and swatting away whoever touched him while trying to hide his neck. He wouldn't be the most ticklish person ever but if he's caught off-guard it be easy to get an attack in and have him wheezing from laughing too much, although he would definitely seek revenge afterwards.
Nagisa: The most ticklish part would be his feet. The moment somebody starts attacking him with tickles he'd be incredibly loud, his laughter would be a mixture of laughs and screams and he would certainly be kicking all over the place and someone might or might not get accidentally kicked in the face during the tickle attack.
Rei: He'd be too proud to admit he's ticklish so the discovery of his tickle spot would be completely accidental, also since he'd be embarrassed to admit that it's his underarms.
Nitori: Nitori would have a really ticklish tummy and sides. He'd be really squirmy when someone starts to tickle him, he'd even shed tears after a little while. Depending on who's tickling him, he wouldn't really enjoy being tickled but he wouldn't hate it either.
Seijuro: Surprisingly enough, he wouldn't be ticklish, no matter how hard anyone tries, they get no reaction when they start to tickle him.
Momotarou: Momo would have the misfortune of being ticklish practically everywhere. Growing up it might've been a problem since it would be used against him, especially to win arguments and especially if it's his brother.
Sosuke: His upper arms, all the way from the elbow to his shoulders. It seems quite strange, but rub his arms with a feather or a piece of paper and it'd make him squirm, although he might end up with a scowl on his face.
Kou: She'd have really ticklish legs and she wouldn't be too fond of the tickling sensation. She'd feel more if she's standing next to something that might touch her leg, like a plant or a loose string from a skirt or shorts more than actually being tickled.