PS: Also there are some platforms that aggregating stats from all over the YT. They can be really helpful to analyze your traffic and make farther conclusions. For example, you can see your competitors and see what are their adventages comaped to you. I dont want to leave a link here so PM if u are intrested.
I found fourms ontopic with your content to be helpful starting out. but its also important to be active and engaged in those community's as well. no just come by every now and then to drop a link and maybe reply to one or two other topics. if you do that it won't likely be well received. but really you should concentrate your effort from within the youtube platform that is really where your growth will come from.
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.