I would be lying if I didn't say it was nostalgia, but I think that my favorite was Wild World. There's just such a feeling that I get that can't be replicated even with the new and improved music - falling asleep while playing my DS at 3:00AM, playing with my friends online for hours on end, chatting about nothing... writing silly long bulletin board messages to one another, getting yelled at by Resetti because I fell asleep while playing... I love the other soundtracks, but none come close to the feeling I get when I hear 12AM hourly music, talking to whichever grumpy villager happened to be up... Wild World was with me even when nobody else was, as clich? as it sounds.
I prefer New Leaf music over the old music, but Wild World definitely has a very nice soundtrack. Some of New Leaf's music I don't like and the same with Wild World.
I like Wild World's best, mainly due to nostalgia and how great listening to it makes me feel. But honestly, they do a great job on every game's soundtracks.
Let's Go to the City has the best soundtrack, in my opinion. The original Animal Crossing comes pretty close as well. They're all great in their own ways.