There isn't really a best as it depends on what you're trying to do.
If you're just starting out, Facebook or Twitter as they have the most personal feel which means you can just message people/respond to them, give them a lead magnet, and get them on an email list.
Start getting 10 emails per day after 1 month, you will have a targeted email list of 300 people and should start to be able to make a few sales.
Month 2 you do the same and once you got 600-1000 you can start to use your email list to grow your YouTube channel, a blog or maybe your own Facebook group something along those lines.
Starting with one channel is absolutely fine, but you have to build an email list. Period.
Also, AM is a business the first 1-3 months when you are learning, you might make a really small amount of money, but you should just aim to learn in this period, you should make some, especially if you stick to the 10 per day emails, but it will be a very small amount.
Once you start learning what converts best, it will become much faster.