Reddit, because it already has a crypto space in it so more chance for conversion for those who are acquainted with crypto while still having a large user base who are yet to be exposed/enticed.

Youtube and facebook may have bigger audiences but I feel like conversion would be harder to do without the right message and near frictionless onboarding.

The true right answer though is split it into all 3 and watch the numbers and adjust as you go while tailoring the ads for the platform for maximum efficiency.