If I put them in order, here's my picks.
1) Great Service: Don't cruise to be waited on hand and foot, but a little pampering certainly is nice : ). A sincere smile and a simple hello from crew and passengers, enjoying either their job or their vacation, truly set the stage for a good cruise experience IMO. A lot of "not the best" ratings can be easy to forget if everyone is friendly.
2) Great Ship: If I'm out to sea, any ship is a good ship. I love to explore the ship, and enjoy discovering every inch of her. Old, new, plain, or ornate, they're all special. Each ship has something that I'll find cool, interesting, or just plain fun.
3) Great Food: Not that I don't enjoy delicious tasting food, but not that important. Don't get me wrong though, I love to eat, and would NEVER miss a meal. But a dining atmosphere means more to me. Be it a early morning bagel snatched from the buffet, eaten on an aft deck overlooking the immense wake with a hot cup of coffee - doesn't get much better than that. Or a 5 course dinner in the ship's alternative restaurant. What's that my kid's say? "It's All good!"