Out of all of Hololive, it's almost undoubtedly true that Koyori would give the best blowjobs.
Sure, some of the other holos might be easier. Once you had her convinced to do it, though, Koyori would be by far the most competent at it.
Let's start with experience. We know that Koyori is straight. She's said on stream what her preferences with regards to men are. We know that Koyori has a good looking body, because she's slim, fashion-aware, and a practiced talker. Koyori is also pretty neurotypical for hololive, with references to going out shopping with friends as a regular activity. All of this points to a girl that would have gone down at least a few times in her life, but you have to remember that she literally worked at a pub while pursuing her dreams of being an idol.
That's the sign of an outgoing girl that's at least been casually intimate as a matter of course. When it comes to sucking dick, she'd know what she's doing. Then there's enthusiasm. Koyori is a perfectionist at using her throat. She's a tryhard at talking, she does her reps with tongue-twisters, and even when it comes to casual activities like gaming and quizzes, she always gives it her all. There's no reason to think she'd give any less when it comes to pleasing her partner.
She would be shameless about slurping on cock, and even continue when it got uncomfortable for her, all just to ensure that she does a good job. And finally, there's raw, natural talent. Koyori has amazing lung capacity and an agile tongue. Anyone that's heard her talking or even heard her cute little noises knows that she's an expert at making you happy with her mouth.
In short, if you want the absolute best blowjob that you can find from Hololive, pick Koyori. There is no other holo that will drain your balls half as well as Koyori. And that's one reason why Koyori is my oshi