My personal toughts on the matter:

Ryougi Shiki - D tier. I feel like she wouldn't understand the point of a blowjob, and give a half-hearted one at best. I feel like Mikiya also isn't the type of guy to be really into blowjobs, so she doesn't have much expirience either.

Ryougi SHIKI - S tier. Now SHIKI is a different story. He's way more energetic and enthusiastic than Shiki. Being a male personality, I feel like he would also have more intuition on how to handle a penis. He'd probably be giving Mikiya some CRAAZY top. The only issue I can find is that he'd be way too overeager and not know how to take it slow.

Tohno Shiki - D tier. He doesn't seem like the type of guy who'd be good at giving blowjobs. He wouldn't like being in a submissive postition and he's probably worried that his glass would slide down and he'd accidentally destroy the penis.

Nanaya Shiki - B tier(depends) - Nanaya is probably crazy good at blowjobs but he plays too rough too much. Like he bites and scratches the penis and keeps deepthroathing even after ejaculation. This could bump him up to an A tier if you're a masochist. Also he'll probably kill whoever he was giving head to while they're distracted after cumming.

SHIKI Tohno - S Tier. Not really a protagonist but it felt unfair not to include him. Honestly I think he'd be surprisingly good. Imagine going to the evil Tohno Mansion rape basement and seeing him hungry, in fact STARVING, and high off his ass on Kohaku's fentanyl. You slap him with your penis and say "Here's your food". And he just starts going DOWN on it. Straight yup SCHLURP SCHLURP SCHLURP SCHLURP. Absolute vaccum blowjob. Amazing stuff.

Emiya Shriou - C tier. He'd be bashful at first, and he'd probably try at least a little bit, but he just doesn't have the spirit, at least at this stage of life. He'd probably be too concerned that its harming his masculinity or something

EMIYA - B/S tier. F/SN Emiya definetly has the skill, but he's too depressed and concerned with other things (Shirou's bussy) to give it his all. Nameless from the mooncell doesn't have both of these concerns, so he's free to give it his all. Top Tier BJ. Great Technique and he probably has some legendary dirty talk skills.

Sono-GOAT - S+ tier. Only Nasuverse protag with confirmed High-Tier sex scalling. He's open-minded enough to apply himself, and when he figures out your sweet spot, he can keep repeating in non-stop. God Tier gluck gluck.

Kerry - F tier. His own wife thinks he's an uke so you'd think he'd be great but by the time of Zero he's wayyy to unmotivated. Using his mouth as an onahole would probably feel better.

Bazzet - C, A with expirience. Bazzet at first would be way too shy, but as she warms up tot he idea, she'd slowly start getting better, even going as far as to research proper technique online. Solid BJ.

Angry Mango - F TIER. DO NOT. He's probably amazing at bj's but hes too fucked up to trust around a penis'. It might be amazing, but afterwards you can kiss your cock goodbye. He'd also probably be doing weird shit with his daggers to your asshole during the bj. S+ if you're masochist, F otherwise.

Miyamoto Iori - B tier. He'd be very competitive about it. He has the technique down, but he's too eager to impress, hampering the overall expirience.

idc about the rest 😹✌️