I'm bored so let's go into a bit of detail.
Instagram is newer and fresher with a simple (before THAT update) layout that is easy to navigate. It's easy to share photos, talk to people, find things you are interested in and so much more. It is more of a social networking site and I really like using it. However, some updates have let the app down in the past and now I think it's just trying to capitalise off of other social networking apps by adopting features identical to theirs (notably Reels and IGTV).
YouTube is the OG and I will always have a soft spot for it. I can search up anything and all the time, I will get a great result. I like to watch commentary and gaming channels and YouTube is the best place for it. On the other hand, it has been overshadowed by it's social media rivals (TikTok) making it less popular than it used to be. Also, many creators have had problems with making money from the site and copyright issues, which is a negative. Oh, and YouTube are getting reeeeeeal bold with those unskippable double ads.
In conclusion, Twitter.