The best way to figure out 'which firm is best for me' is to drill down into what your needs really are.
For example, is it really necessary that a single firm does the design, branding, development and everything? A full service firm like that is easily found, but they are frequently expensive and there is usually something they are 'best' at (i.e. they are stronger at design than development, or the reverse).
There is also your budget to consider. Some firms will happily eat up your whole budget, but you can save a ton of money by separating out a few things and looking for multiple vendors to handle them. For example, hiring a freelance designer to do the logo/designs, and a ui/ux freelancer to wireframe it out would allow you to search for a firm that just does development, and that's going to be easier and cheaper (usually) than finding a full service shop.
It's also a good idea to consider the specifics of what you need built, as has already been mentioned. If you have a complex app that is going to really take some technical chops that might lead you to a different vendor than a relatively simple app. Along those lines, don't forget to consider the back-end - most 'full service shops' are standardized on a specific stack (i.e. php/mysql) and that may or may not be a good choice for you.
Finally, consider what you can bring to the table so that you aren't paying for service that you don't need. If you are very good at documenting and are able to write up the specifications yourself, you might not need such a full service firm. If you are new to project management or the whole app world in general, a powerhouse project manager could bring a lot of value but just because a shop offers soup-to-nuts service doesn't mean they are good at project management. So many of my clients are refugees from the soup-to-nuts shops, actually.
Look in the mirror and try to define your needs in more detail, and you'll have a better idea about what that 'perfect-fit' vendor might look like, and it'll be much easier to find them!