Since the utility of social media is different for different types of B2B businesses, I have 2 favorites:
1) For a B2B businesses my favorite social media platform is Linkedin, simply because it can be used for getting in touch with business owners.
2) For B2C my favorite social platform is twitter. This is because people across the world use Twitter and many famous influencers of every industry are pretty active on twitter. the facility of posting images and videos gives a feather to the hat.