Unfortunately Sorting, only applies to user and goes back to "Most Relevant for others.
Here are what seem to be the top 5 factors in what makes a review more relevant.
Length - The number of words in a review had a clear impact on how relevant it was considered. There was a consistent correlation between the number of words contained in a review with its position in the Most Relevant rankings.
Keywords - Using the name of the business in the review led to an increase in the post’s relevancy ranking.
Specific - A review that contained a specific story or example was more likely to rank higher than reviews with generic content.
Local - A review by a user who posted other reviews in the local area is deemed more relevant than a user with either no reviews or one with reviews from a different geographic location.
Time - The more time that passes since a review is posted, the less relevant it becomes.
@wlara When you said best, what do you mean? Would you consider 5 star reviews to be the best? I’m just curious.
I feel it’s best that the default reviews is “Most Relevant” for users so people will not get biased reviews when evaluating a location. It’s not always a good thing to put 5 stars review at the top, because that will give impression that a place is better than it actually is (since most people will not bother to scroll down to much to discover the rest of the reviews)
I had some difficult to understand the title of your post, @wlara , as you wrote “Not able to sort the best Google Reviews for a client”. Why? Because this seems to identify you as a business, providing services to other business in Google Maps.
So I wanted to check your profile.
While you can of course provide your services to everyone, using the Local Guides program for branding your business (as clearly declared in your profile) is against the Local Guides Program Terms and Conditions : "Organizations, brands, and businesses are not eligible for the Program.If you offer services for hire (such as marketing yourself as a Street View trusted provider), you are not allowed to bundle such services with your Local Guide membership"
Probably you can better filter the reviews developing a filter through the Google Maps APIs, but this is not related to the Local Guides program