I would say that my "Scandanavia and Western Europe" cruise aboard MV Royal Princess (June-July 2001) was the best. There were other single people on board, and both the ship and the ports of call (Edinburgh, Oslo, Copenhagen, Hamburg, Amsterdam, Zeebrugge, and Le Havre) were absolutely fantastic.
My worst cruise? Hmmm... That call is really tough. I have not taken a cruise that really stands out as the worst overall.
Best cruise: Halloween Cruise on the Carnival Paradise non-smoking ship in 2001. It was good medicine for the post 9/11 period. Grear people, and fun! I'm going again this year.
Worst cruise? 7 night cruise in August 1998 on the Carnival Elation to the Mexican Riviera. TONS of bratty kids. I love kids, but the parents let them run wild around the clock, banging on passenger doors in the middle of the night, adults sneaking alcoholic drinks to kids ( I saw one adult receive a ticket, charged with buying a drink for a minor), saw drunk teens getting sick on the elevator (and not from sea-sickness).
A bad time of the year for singles to go on a cruise unless you know you are going with a LARGE group of singles, lesson learned. Dancing when there were only about 20 singles on the entire ship was NOT much fun.
And the visit to Cabo San Lucas was a waste of time as we arrive at 7am and stay for only the morning. Cabo is an EVENING experience not to be missed, otherwise, it is a waste of time.
Lastly, when I decided to take the sheets off the bed one time so they would be replaced, I was shocked to see a particular spot in a particular place on the mattress! OMG. So I flipped it over, and saw the same sort of spot in the same place!!! That pretty much ruined it for me.