Well, every social network is good in its own way. It mostly depends upon the niche you're (as a marketer) in. For example, for any B2B business, I think Linkedin is the most suitable platform. However, the cost of ads is much higher than what you'll find on Facebook or Instagram. 

If your business is targeting the consumers (general audience), then you should definitely go for platforms like Facebook, Instagram, Snapchat, and Twitter. 

Reddit and 4chan can also be very useful if you know what you're doing. The Reddit audience hates ads, so you need to get creative with what you're trying to promote. Try to be as transparent as possible on Reddit and instead of simply promoting your business, try offering some sort of value for the audience. 

Also, don't limit yourself to social media ads. SEO can also be a very cost-efficient strategy in the long term.