Number one for me (by far) as a client : small groups of divers with homogeneous level and experience, with a small total number of divers on the boat, hence the maximum of flexibility for choosing/optimizing the dives.
For example, much better in Indonesia (Bunaken, Lembeh ...) to be 2 divers per guide and 2 guides per boat (4 guests total per boat) than 6 divers per guide and 3 guides per boat (18 guests total per boat).
Other example, on a Red Sea live-aboard, much better to be 6 to 8 divers maximum (same level, otherwise it may be a PITA) on a 18 meters boat with one guide and one Zodiac, than 24 divers on a 35+ meters boat with 2 guides and 2 Zodiacs.
Real (and worthwhile) luxury/service in diving is not plasma screens, or marble in the bathroom ; it is to have something akin a private boat/dive center.
PS: apart from this, having good, safe air (or gas) fills, and having oxygen readily available, is of paramount importance. But safety stuff like this isn't part of the "value of use" of a Dive Center. I mean it should be guaranteed to the client anywhere and not be part of a "Best Service". In other words, safe fills is a constraint for the dive center, not an incentive for the client.