Each cruise that I've taken has had a memorable element. We cruised through Italy and Croatia and the most memorable part of that cruise was sitting on our balcony while the ship set sail from Venice. If I could choose only one though it would be the 30 day repositioning cruise that we took on Norwegian. We sailed from Seattle up to Alaska and then back down through Oregon, California, Mexico, Costa Rica, and Columbia but the highlight of the cruise was the passage through the Panama Canal.
We've cruised Celebrity and Norwegian and I think that each has something different to offer. While both ships do a good job of entertaining you throughout the day, Celebrity in my opinion has better port of call presentations and the food is much better but I would pick Norwegian for evening entertainment.
This is such a wide open question. All cruise lines have positives and negatives, so it's really just a matter on focusing on what YOU want from a cruise. Some lines are very family friendly, others focus more toward older adults and some provide a younger party vibe. A couple more economical lines that I consider good are Royal Caribbean and Princess, while Cunard and Crystal are more upscale. My favorite line is Disney..they have little bathtubs in the staterooms (others do not) and I like the quality and family friendly atmosphere. They also have a nice adult area. My husband and I often travel by ourselves without our grandkids and still find Disney our favorite. Another option is river cruising, which is getting more poular all the time, and with good reason. I'm a travel agent and contribute a monthly column here. I've written a couple of articles on cruising, so please feel free to check them out.