Depends on what you want it for, if you are looking for quality traffic then facebook is the best but if you are just looking for a lot of traffic to boost your alexa rankings for example, then twitter is the best as it is easier to get traffic from
Very heavily depends on what you're going for. What are your goals? Promotion, traffic generation, or just general social networking?
At this time Google+ is best for social media many people that move in to Google+ Many latest feature appear in Google+ at this time search engine love Google plus. If you talk about Facebook maximum 40% profile FAQ that not good for your personal status that is the reason people move in to Google+ that effective for the user.
I do like both. In various situations i will be using both for posting our company/product/service updates.