Mostly singular, but it could be plural in some cases (depending on the intention of the speaker) and there is enough ambiguity that treating it as either singular or plural is unlikely to cause problems. Both singular and plural may be acceptable.
So what is going on here. Normally adjectives can precede nouns with no "of", so this means that the superlative has actually been "promoted" from an adjective to a noun. "The most intriguing" means "The most intriguing one", or possibly "The most intriguing ones". Here you can see the ambiguity in number. Both "one" or "ones" is possible as the intended meaning.
Here is a much simpler sentence that shows the same ambiguity in number.
The best was John.
The best were John and Richard.
A superlative adjective "best" is promoted to a noun. But it can be treated as either a singular or a plural noun. In this case, the number ambiguity is resolved by the complement. If the complement is John, then "best" means "best one" and is singular, etc.
But, in your more complex sentence, there is no way to resolve the ambiguity in the number of "the most intriguing". It could be either singular or plural. In the first case, the verb implies that it is actually plural, in the second it is singular. Singular is probably more common, given the usual meaning of superlatives.
The plural "efforts" is a modifier, and it doesn't change the number of the noun. But it's proximity to the verb can cause interference. Especially when, as here, the number of the main noun is ambiguous.