We've enjoyed Royal, Carnival, NCL, and Princess. (Around 15 cruises, in our 40's)
Princess is great for Alaska but boring for the Caribbean. My kids do not want to go on Princess again. It was just my husband and me for Alaska, and we thoroughly enjoyed it.
We got a last minute Haven booking on NCL at a great price so I don't think I can make a fair comparison to Royal and Carnival, but Haven is an amazing "step up" if you can score a deal. It was our first NCL, but we will be back if we can do Haven again. The shows were better than Carnival, and we loved the ship within a ship.
For our 25th anniversary, my husband and I would like to take our first Mediterranean cruise. What line would this group suggest? We are ok with wearing country club casual for dinner and would like a bit of luxury, but we are not pretentious by any means. We enjoy comedy shows the best on cruises, but enjoyed NCL's Broadway shows too.
