Priorities:
Location, location, location. Not meaning it has to be popular, but meaning it should be a good port: protected from the elements, a reasonable approach and reasonable proximity to places I want to go.
Next, it has to be in reasonable
repair. I don't want wobbly docks that one nearly falls in the water while transiting or lack proper protection or power/water fails continuously.
Shore toilet/shower facilities (guests really like a break from showering aboard, sometimes I do too)
Desirables, but not deal killers:
Onsite service staff (but they need to be good or are just an impediment to getting the right talent)
Parking
Onsite
fuel/pumpout
Basic parts supplies available (oil, filters, clevis pins)
Friendly staff
Mix of power and sail, which seems to allow for better overall comradery when there is a balance of power.
Entirely optional:
Pool, restaurant. Nice to have, but don't care.
No control whatsoever:
Price. Around here, you generally get on a waiting list to get even some of the above requirements, so you pay the going rate or you don't. I know of a couple of untouchable marinas that price strictly for the uber-rich transients, but I woudn't want to be there permanently anyway.