Hmmm....that's an unusual question.
I would think that the priority in choosing a cruise line would be where they cruise to, type of ship, clientele, cost factor, maybe even registry nation of the ship before you'd choose it for the benefits. Since cruising is something most people do very infrequently (I'd think that cruising once a year is way more than most people average), I never thought to choose a cruise line for their repeat customer benefits.
I belong to 2 frequent cruisers clubs, and so I will tell you what I know about those benefits:
Celebrity Captains Club:
After one cruise:
*one category upgrade
*presail specialty restaurant reservations
*custom air request
*Captains Club party
*welcome amenity (usually a fanny pack or tote bag)
*spa gift with purchase
*casino coupons
*discounts on golf clinic and simulator
*complementary wine tasting
*reunion cruise
*newsletter with exclusive offers
After 5 cruises:
*all of the above
*priority embarkation
*private cruise event
*behind the scenes tours
*preferential debarkation
After 10 cruises:
*all of the above
*priority status for shore excursions
*prioirity tendering in port when needed
*private shipboard departure lounge and continental breakfast
Royal Caribbean Crown and Anchor Society:
After one cruise:
*coupon booklet for on-board activities
*complimentary wine tasting
*welcome back party
*commemorative gift (fanny pack, tote, toiletry kit, etc.)
*cruise savings certificates
*dedicated website
*member magazine
After 5 cruises:
*all of the above
*private departure lounge
*onboard event (party)
*robes in room
*priority check in
*dedicated phone line for pre-cruise
*past cruiser discount on balconies and suites
After 10 cruises:
*all of the above
*priority wait list for spa and shore excursions
*priority departure
*concierge services
*priority specialty dining reservations
After 24 cruises:
*all of the above
*personalized amenity
*behind the scenes tours
*preferred dining seating
*Loyalty Specialist for cruise planning needs
*luggage tags and lapel pins
Additionally, I know that Royal Caribbean has a Concierge Lounge on board some of its largest ships for those staying in full suites.