It depends on the products and services you are selling as a business entity. However, in general, I believe that personal selling by means of a sales force is the most effective form of promotion. This applies to selling products, whether at the wholesale level or the retail level. It also applies to selling services - business to business, or business to consumers.
In running different retail food stores, I found that the personal touch and daily face-to-face interaction with customers generated the most goodwill and eventually increased sales for my businesses. There is something to be said for engaging in conversation with customers to learn and understand their wants and needs as relates to products and services you offer.
While other forms of promotion are valid and beneficial, having a sales force that engages customers is the best way to get a read on what customers expect from your business. Verbal and non-verbal communication from customers allows you to understand their needs from their point-of-view, without you having to guess at what they really want from your business.
Personal selling by a sales force promotes asking questions of your customers, and they of you. It is direct conversation designed to come to a mutual understanding of how best you can serve your customers. Personal interaction between employees of your business and customers helps fruitful long-term business relationships develop, whereby you garner repeat business based on the level of trust you've developed with your clientele based on conversations - in person - with them.