What do you call the linguistic ambiguity in an assertion "Blah is the Best X?
Solution 1:
I am not aware of a specific term for this type of ambiguity. I would actually argue that the generic form you present is not inherently ambiguous:
Spot is the best dog.
Spot is the best dog at running.
These two sentences have drastically different meanings and I do not think it is accurate to suggest that the former could ever imply the latter without some additional context.
The only time it is ambiguous is when the object holds some specific quality:
I am in the best class!
This either describe (a) a group of people who are "the best" or (b) a specific course that is "the best". This ambiguity is nothing more than a word having two meanings that both satisfy the semantic requirements of the sentence.
Another example of this problem:
"I ran out of the doors," said the carpenter.
"I ran out of the doors," said the athlete.
One of the most famous variants has its own Wikipedia page:
Time flies like an arrow. Fruit flies like a banana.
Unfortunately for you, the most relevant description of this phenomenon is syntactic ambiguity:
Syntactic ambiguity, also called amphiboly or amphibology, is a situation where a sentence may be interpreted in more than one way due to ambiguous sentence structure.