If people are geeky and into "high tech" (like Dave Beers) then I often recommend Royal Caribbean.
If I think a person is a "people person" like Linda then I might recommend Carnival.
If I feel a person is strictly anti-regimentation then I might recommend NCL.
If a person is a "traveler" I will recommend Oceania or Azamara.
If they have a taste for the finer things in life I will recommend a luxury line - exploring all the differences between them, like "which is more important, quiet solitude or a bigger variety of activities. Do you want a butler (then you have to explain what they do), then you ask "what's more important, a comfy stateroom, or a beautiful ship? (as in 'where are you more likely to spend your time, in or out of the cabin?').
If they want to "travel" and like quieter ships I also explain river boats. This is also an entirely separate conversation.
If they have kids I will tell them about Disney, but then I will also explain how much more expensive they are and that you have to book early. If they have "families" that are not just kids then I will mention Royal Caribbean and NCL (for their fuzzy characters but also more adult activities).
If I feel they are grown ups who enjoy indulging their own inner child (like my wife) I might recommend Carnival because they have the most interactive staff members and activities - fun for all, whether you want it or not.
If they seem like they DON'T want any childish antics I go towards Celebrity, and Holland America, depending on budget and other factors.
If they just want a good solid "cruise" that does everything well but doesn't necessarily stand out in one specific area I might say "you can't go wrong with Princess."
If they want to experience the "classic ocean liner" experience I go with Cunard.
If they want the "yacht" experience I go with SeaDream or Windstar, or the smaller Seabourn or Royal Clipper.
If they are single and younger; Norwegian Epic, older; Crystal. Or I have been reading up on Grand Circle more lately. They have a large single following (up to 25% to 40% on many sailings)
Feel free to add your own...