The
best digital marketing agency is one that is well matched with the consumer, not a 'one size fits all' kind of work.
First of all, their height. The companies set up to manage accounts of millions of dollars are very successful and hire many top individuals. For a small business, they will be a bad alternative because you waste a large sum on project admin overhead that you don't need. The opposite is also valid - try giving a small boutique business a record agency and the lack of size will cause no end of friction.