My favorite is day 1, since whenever I hear it I think of the excitement I felt when playing New Leaf for the first time. Also it's even better with headphones.
Gave them all a quick listen to jog my memory but I'm feeling:
#1: Day one - a nice backdrop to seeing new 'small-town' sights for the first time #2: Day three - I find it meshes the best parts of one and two together. Also, love the softness of the track near the end #3: Day two - 4:00-5:00-minute mark is gorgeous, but enjoyed the others more overall
In my opinion, day 1 sounds the best, it almost sounds nostalgic in a way. My second best would be day 3 since I still feel like it's nice music for Main Street. Day 2 kind of sounds incoherent to me, still a nice piece of music but a little strange for a theme in Animal Crossing so I'd put it in last place.
I love day 2!! Its so weird how day 2 sounds all fancy and completed yet day 3 is the final one you get. I think it should have been day 1 - 3 and then 2. But I PERSONALLY like 2 the best because I think its just perf. I love all of the mainstreet themes because they are special in their own ways! I do not know if this works on newer towns that are not on day 3 yet but if you wait after a while day 3 stops and the game will play day 1 and 2. After it finishes day 2 it goes back to 1 and wont play 3 again. Neat detail!