This is a *very generalising sweeping question*, though if there was only one promotional tool (eg brochure, business card, twitter, poster, website, TV advert etc) that could be used for a business client, what would you recommended?
Facebook & social media are great if done right, but they are opt-in - target has to like or follow them. I've seen businesses set up Facebook pages that no-one 'likes' , who wants to like Jims Butcher Shop & every other business they might shop at? before they've even shopped there.
But I don't have an answer to your question about what's best. It's a little bit 'if you had to kill everyone in your family except one, who would you save'.