The best cruise I had was really my very first, at 6. (There, I said it, 6.) It was on the former Carnival ship Tropicale. The customer service was first-class, the food was the best so far among my past cruises (due to the executive and sous chefs being French), and the amenities were awesome.
Second to that was my second cruise with Disney Cruise Line, on the Disney Wonder. Despite the fact that it was merely a 3-day cruise, I found the food and service better than when I sailed on the first year of the cruising company's operation. (That's when the Disney Magic was the first and only ship at the time, before the Wonder came along.)