There will be no single presentation design agency that is right for every customer. There are enormous differences in capabilities, price points, expertise, global reach, approach, and many other factors. There are some presentation companies that just make adjustments here and there at ultra-low rates and some big firms that charge exorbitant sums.
Try to compare the capabilities of all the agencies that you shortlisted. What kind of storytelling did they employ in their past work? Was it intriguing? What was the messaging like? Impressive enough? How big is their team, and how experienced are their designers? And once you start engaging with them, make a keen observation of all their processes. What was their approach like? Was it collaborative? Ask for the portfolio, especially relevant work examples. All these factors are a good starting point to help you pick a good presentation design agency.
Figure out what you need, set a budget, and then start reaching out to agencies that fit the bill.