Best retirement was probably Liberte which had the distinction of bowing out with dignity while still close to the top and meeting a mercifully quick scrapping death. Worst fate, cumulatively, are the vessels (naming no names for legal reasons) which refuse to die, and go from First Class Liner, to Tolerably Well Maintained But Older Mass Market Cruise Vessel, and then from there fall into the tarry black abyss of Budget Cruise Liner. Fortunately, SOLAS has made those a dying breed, at least in US waters, but there is NOTHING more depressing than being aboard a liner one had been on in 1973 (when she was a flagship) 23 years later and discovering that she was showing her age, badly.
Leaving aside those liners that met an end involving loss of human life, I'd have to say the one that always rather saddens me is the way the Oceanic II finished. Long before her time, put to a use for which she was manifestly unfit (patrolling the Shetland Islands).
I also feel rather sad about the last years of the Medic - as a whaling factory ship named Hektoria, and then torpedoed while serving as an oil transport in WWII.
Worst retirement would have to be a triple place finish; The T.V. Michealangelo, T.V. Raffaello, and Queen Elizabeth (Seawise University). Best retirement, probably the Queen Mary. Hopefully United States will run again. Plenty of other tragedies in terms of wrecks and losses, however.
I suppose a case can now be made for the Oriana which looked set to spend a long retirement as a tourist attraction in China. A rather nasty storm managed to sink the ship at her berth. She was refloated but after survey, was deemed irrepairable and she was towed away to the breakers.
The Caribia, I forget the line, I think it was Cunard's Caronia at one point. Anyway, the Caribia spent a decade trying to scavenge up a buck. Failed, and struck a breakwater in Guam on the way to the scrapyard. It capsized and broke into three pieces. Now that is a horrible life right there.