Word of mouth is most likely the most trustworthy source of value. People are much more likely to trust a friend's opinion than a billboard's. That being said, companies cant rely on people to talk about their products where everyone who looks at an ad gets the message. The impact is greater, but fewer people can be counted on to be exposed. However, over time, a brand will increase exposure
Although ads and signage are both great ways to advertise, word of mouth is the most effective. People are much more likely to try out a service when they have friends or family who tell them about a positive experience. Word of mouth increases exponentially as people try out the business and then tell people they know.
I believe word of mouth is the best advertising but even word of mouth advertising has been degraded with the on set of the Internet. I use to be able to trust review sites on-line now, but now you can't trust those because they or the products company is likely to pay for fake reviews. I trust in person word of mouth mentions only.
I believe that word of mouth is the best advertising although it might not be the most far-reaching advertising. When a person can personally vouch for how good a product is that will likely impact another person more than just seeing a stranger on TV vouch for it. We trust people we know and are more likely to buy products that they endorse.