This is a very difficult question to answer. Best probiotic for what? Do you have something specific you are trying to accomplish?
As others have stated, I would say that Gut Health is a top-tier product. IMO, what really puts it over the top is that it is a top product with a value-price. You can get a bottle, which will last 2 months including a loading phase, for under $30. Gut Health has the potential improve the absorption of nutrients, reduce indigestion and bloating, improve your immune system and actually help improve the mucous lining in your gut. I say it has the potential because some people will have already have the gut bacteria that they get in Gut Health populating their system, and the effects will not be noticeable beyond just not getting as many colds and such. These people, generally, won't have major issues to fix to begin with though. Some people may start taking GH and the bacteria may actually start creating issues in their gut for a week or two - because they didn't have the bacteria and the change in their microbiome will upset things and they will need time to adjust. These people often see the most benefit if they stick it out.
When Gut Health was designed, Matt considered making it a XXX Billion CFU product with 20-30 different strains - but this become problematic. A lot of the bacteria's don't play well together and if you have 30 strains in a product, they fight each other to the death. By the time you take the product, strains will have been killed off, become unstable, taken over, etc. It makes it difficult to create a viable product that is predictable in what you end up getting. Plus, some strains work against each other in their effects on the human body.
Beyond probiotic and prebiotic funciton, Gut Health also includes ingredients that help build the mucous lining in your gut - as I said above - which is often overlooked in such products. If you have IBS, leaky gut, or even ulcers - these ingredients will potentially thicken the mucous membranes and protect your stomach and intestines better than a probiotic alone.
IMO, Gut Health is a great formula that contains what most people need and also provides a great "base" for people who may want to try individual strains to combat a specific problem - like S. Boulardi or Reuteri, etc. But most people won't need to add anything to it unless they have something specific they are trying to achieve.