what do you think is the most effective channel for acquiring new customers ?

When you are starting a new business, "The direct" is the best. Its very cheap, less risky and tells you more about your customers. After you have a few satisfied customers, you can go for "The referrals". Later on as customers increase you can start building your "Social profile" collecting reviews and feedback from these happy customers. You should also plant a seed of "The organic" here. After you have some digital proof of customer satisfaction, you can go for "Advertising" and promotions at larger scale. Also by this time "The organic" seed will grow into a tree and will start giving fruits and will be your main source of sales.