Brian, the attached from Laura Lawless may help. See especially the introductory paragraphs and the final example regarding ‘the best restaurant …’. 

The first sentence quoted is a superlative statement of opinion regarding something that occurred to a 3rd person - the subjunctive indicates that there is ‘doubt’ the 3rd person would agree with the statement, that the speaker does not know “for certain” it was that person’s best holiday. 

The 2nd sentence is a statement of personal experience. The indicative here informs that the speaker is not seeking to be hyperbolic - not saying the singers are the ‘worst ever’, or the ‘worst in the world’. There is no doubt to the speaker that the singers are the worst he or she has personally heard - a statement of ‘fact’. Note that it also does not propose something like ‘the worst I will ever hear’.

It is the speaker who decides what is factual and what is doubtful - native speakers will generally therefore ‘choose’ appropriately between subjunctive and indicative to convey to the listener the nature of their statement, notwithstanding that in most situations the superlative ‘may’ give rise to the subjunctive, regardless of speaker.

https://www.lawlessfrench.com/grammar/superlative-subjunctive/

Cécile, do you disagree with Laura’s discussion ? It seems from her article, that the 2nd case could be either as it is the same structure as her final example. 

Thanks Cécile - missed the example in the lesson when I scanned. Context, context !