If you are looking for the best public relations firms, there are a few criteria which can help you to narrow down the search. You will want to figure out what the role of the public relations firm will be in your overall marketing strategy. If you are interested in sending a certain type of message, then you there are different PR firms which may have more expertise in that matter. If you, however, want to know what will be the best public relations firms to handle damage control, then you will want to work with a PR firm that has a lot of experience with countering negative appearances and reactions. More often you will find that certain firms specialize in certain fields of public relations.
You will also want to consider the process of finding the best public relations firms to be something akin to a job interview. You will want to see examples of previous work, speak with references, and generally learn more about whether or not it will be the best public relations firm. The more that you learn about the PR firm that you are currently in talks with, the more you will realize whether or not they are the best public relations firms for your company to work with. Get a firm understanding of their client history so that you can determine whether or not they are what you are looking for, and you will also be able to determine whether they fit the first criteria as well. Often times, their work with clients will be reflected in the needs of that client; research the public perception of these past clients as well so that you can effectively discover if they will be the best public relations firm for you.
You can always ask them up front if they are the best public relations firms for your company, but the trick with PR firms is that they make a business of talking up how great their clients are and spinning any and all information towards the end. That makes actually speaking with the PR firm themselves something of a PR campaign. You will have to use different means to cut through the positive image boosts and get to the truth of the matter, but the best public relations firms will no doubt be able to provide you with the right service.