I personally am of the opinion that New Leaf is the most enjoyable Animal Crossing to date. I never played the Game Cube version (gasp). I know, I know, I'm being blasphemous here, but oh well. So, I only speak from the experience that I have.
Wild World was great. It was my first true love. However, it lacked in the variation and fun of events/activities to perform. That didn't keep me from investing hundreds of hours in the game, but I think New Leaf brought one thing to the table that would have pushed Wild World over the edge to greatness; mini games, and LOTS of 'em! The island brings so much variety and fun to break up your day, it's great.
City Folk was really just "meh" in comparison to the other franchise titles. Sure, it looked great, but that's about the only thing that was an improvement for the game to me. It still did soak up hundreds of hours of my life... but I can see New Leaf eating THOUSANDS.
So yeah.