I have yet to see a truly satisfactory harem but I imagine it wouldn't exceed 2 or 3. MC's with more always suck hell most harems suck someone always gets left behind in the love department. I would say a barable harem for me if I remember correctly was that of the renegade immortals though it is debatable whether that should be included.
Upon what basis? Diversity of species, rank, colour, size and age, variety of temperaments, completeness of female archetypes, harmony and camaraderie among members, loyalty and utility to the protagonist, multifariousness of kinks, fetishes and sexual preferences, or a medley of all above? Assortments covering all bases, for the exceedingly greedy protagonist. A harem entirely made up of legendaries and shinies doesn't sound bad either.
MC from the novel Not super robot wars. Since its a fanfict I know the girls in its OG series. The characterization is solid. its not all smooth and theres tons of inner conflict as it should be for a harem series.
Hmm, I would say the harem from The world of Otome Games is Tough for Mobs, but that's just my biased opinion, I don't know what others think.
Probably Isekai Nonbiri Nouka. There's variety in "types" with some being very strong and they have a variety of useful skills between them. More importantly, they all make sure the protagonist can do what he wants to do while not losing sight of their own hobbies and goals. To be honest by the most recent chapter their life together is probably better than the majority of monogamous marriages and still realistic enough given the circumstances that I feel willing to make the comparison.