The big 3. They are Facebook, Twitter and Google My Business. Facebook is the 900 pound gorilla. It's a necessary evil. Twitter is great for search engines and getting your site found by them. Google My Business is not really a social media site but, is still included in our Social Media Management Service as well as our Basic SEO service.
Google My Business is a bit of a crossover business profile as it has some social media aspects and some search engine optimization aspects. For these reasons it is by far and away the most important.
You may also consider Instagram and Pinterest. These are both very visual social networks that revolve around pictures. So if your business has a lot to "show" these may benefit you.