This has been answered quite often here. Ultimately, there are no "best." Choose the ones that work for your type of content. Best case scenario is to dip your feet in as many of them but learn to distribute time based on effectiveness for YOU. Reddit was effective for me early on but I never touch it anymore after it became less useful. Also, the smart option is to not treat your other social media platforms as an extension of your YouTube channel. Use them as their own platforms and capitalize in maximizing impressions and views that work for those specific platforms instead of trying to funnel people to your YouTube channel from there. Then you can intertwine them in whatever reasonable fashion you deem to use it.