Всё зависит от смысла.
Всё по общим правилам:
Не совсем о примере, но об общем подходе.Look at it this way:
There is a group of gyms that you classify as "Good"; another group you classify as "Better"; another group you classify as "Best".
So any given gym can be among the good ones, among the better ones, or among the best ones that you've seen.
For some reason, we can't use good by itself without ones, so you can't say one of the good I've seen. It has to be one of the good ones I've seen.
But either better or best can be used without ones.
It's not one of the better (ones) I've seen.
It's not one of the best (ones) I've seen.
So it's not a matter of grammar, but a matter of telling the truth.
If you would classify it only as good (or less than good), then it's not one of the better ones you've seen.
If you would classify it as one of the better ones, but not one of the best ones, then it's not one of the best ones you've seen. (Of course, you always mention the first group, moving upwards in quality, that this gym is not in.)
In short, use better or best depending on which you actually mean!
CJ
Ещё это можно глянуть:
https://forum.wordreference.com/threads ... e.2505602/
https://ell.stackexchange.com/questions ... f-our-most
https://www.phrases.org.uk/bulletin_boa ... s/801.html