The answer to that question is really your decision based on what is the best cruise for you, considering your age and the ages and types of people you would want to be on a cruise ship with; how formal vs casual you want to be; whether you are an active person who wants lots of activities on the ship or would prefer to just relax and not be bothered with doing "stuff"; and how much money you want to spend. NO, all cruise ships and cruise lines are NOT the same.
The very high end cruise lines, which some call the "best" are cruise lines like Crystal, Silver Seas, Regent Seven Seas, and Oceania. All of these are top notch on service and amenities on their ships and also are very formal. In addition, a cruise on any of these cruise lines will cost about double what you may pay on the next tier of cruise lines. But, you get what you pay for if one of these is "best" for you.
I have been on more than 25 cruises on Princess, Celebrity, Holland America, Norwegian, and Royal Caribbean ships. These are the next tier of cruise lines that most people cruise on. You need to know that there are some differences that you need to be aware of in making your selection of which is "best".
Princess, Celebrity and Holland America have great ships are more classy and reserved. They are just a little more upscale and luxurious than the other cruise lines. Passengers tend to be middle aged and up and have cruised multiple times. Holland America has mostly senior citizens (65 and up) cruising on its ships and very few kids and young people.
Norwegian cruise line is good, they cater to families and have mostly passengers in the range of age 35-40 and up. Its the line that you want to go on if you want to be informal for pretty much your whole cruise. They have formal nights like other cruise lines but they do not stress dressing up; they call it "dress up if you want to" night. Norwegian has the most alternative eating places on their ships, but you will have to pay a fee of $10 to $20 per person per visit to eat at some of them. All other cruise lines also have these alternatives but usually only two such places.
Carnival is the budget cruise line that generally has the lowest prices. They cater to young people, first time cruiser and families. Most of their cruiser will be in the 20 to 45 age group and there will be lots of kids on board. Their ships have some good on-board activities like water slides and basketball court. The best of their ships are their newer ships so do pay attention to when the ship was launched is you find a good and really cheap cruise.
Royal Caribbean is also a cruise line that caters to young cruiser and families; ages about 25/30 to 60. It has the advantage of having the most on-board activities, like ice skating, roller skating, mini-golf course, a basketball/tennis court, the rock climbing wall, and on the new Oasis and Freedom Class ships the surfing simulator, the Flowrider. The RC ships that you should look for cruises on are the RC's Oasis Class, Freedom Class, and Voyager Class ships.
Cruises to the Bahamas and Caribbean of 3 to 7 days generally have more young people and kids. Cruises longer than 7 days and to places like New England/Canada, the Mediterranean and South America have fewer kids and more older cruisers.
You must pay for excursions from the ship but you don't have to book excursions and can get off and back on the ship as much as you want in port stops. The excursions available will depend on the port so is pretty impossible to just list the best ones, they vary by port.
You pack casual clothes like shorts and swim wear, evening clothes like slacks, shirt with a collar, and regular shoes, and then formal wear for the one or two formal nights on the cruise. Formal can be anything from a tux or gown for women, to a suit and tie for men and a party dress of women; or just wear your Sunday best.
When you cruise the activities will be there for you to enjoy, just pay attention to what is happening and do what you like. The ship will provide you a newsletter each day with a listing to the time and place for all activities on the ship; read it and go.