It is so so hard for me to rank as I think each of the games have their own special charm, however I'm in the minority that voted for City Folk. I'm a tad biased in that it's the first one I ever played, but I have also played every game in the series (recently). The overall aesthetic is cooler than its counterpart Wild World and the music is just awesome. Plus, CF and NL have almost exactly the same furniture and they both have the best furniture in the mainline series bar none.
However, Wild World has the best writing and dialogue in the series and the villagers had never been more alive than in that game.
However, Wild World has the best writing and dialogue in the series and the villagers had never been more alive than in that game.