If we're talking about the ost, then I really like the Nighttime music when it's snowing/raining, also the main them from gamecube and the movie. K.K. slider songs I love are K.K. Bossa(Which played in the movie) and Go K.K. Rider x3
I love the snow/rain in ac though it feels great to play with that music playing x3
Regarding NL, Brewster's Caf? is one of my favourites. I love that building so much, it's so homely and relaxing. But 1am is definitely one of the best hourly themes imo, along with 8pm and 5pm. I kind of like the music in Kicks too.
I also really liked the upbeat version of 2am from Wild World that was used in Smash Bros. Brawl. Reminds me of sparring with my friends when I played that game somewhat seriously, good times.
Hourly themes
1: 4PM from Animal Crossing (GCN)
2: 7AM from Animal Crossing (GCN)
3: 8AM from Animal Crossing (GCN)
4: 2AM from Animal Crossing (GCN)
5: 11AM from Animal Crossing (GCN)
6: 5PM from Animal Crossing: New Leaf
7: 4PM from Animal Crossing: Wild World (also appeared in the movie as Margie's Cherry Pie)
8: 5PM from Animal Crossing (GCN)
9: 1PM from Animal Crossing (GCN)
10: 12PM from Animal Crossing (GCN)
Building themes
1: The Roost from Animal Crossing: The Movie
2: Able Sisters from Animal Crossing: The Movie (it's actually slightly different than Wild World's/City Folk's/New Leaf's)
3: Able Sisters from Animal Crossing (GCN)
4: Nook's Cranny from Animal Crossing (GCN)
5: Shampoodle from Animal Crossing: Wild World, City Folk and New Leaf (was it even in Wild World?)
6: Town Hall from Animal Crossing: The Movie
7: Town Hall (day) from Animal Crossing: Wild World and City Folk
8: Re-Tail from Animal Crossing: New Leaf
9: The Museum from Animal Crossing: The Movie
10: The Museum from Animal Crossing (GCN)
K.K. songs
1: K.K. House
2: Forest Life
3: K.K. Swing
4: K.K. Condor
5: K.K. Island
6: K.K. Technopop
7: K.K. Metal
8: Rockin' K.K.
9: K.K. Country
10: K.K. Ragtime
Other themes
1: Mr Resetti from the Animal Crossing video game series (I liked the movie rendition of Resetti's theme but not as much)
2: Title screen from Animal Crossing: Wild World
3: Title screen from Doubutsu no Mori e+/Animal Forest e+
4: The Island from Animal Crossing (GCN)
5: Tortimer Island (day) from Animal Crossing: New Leaf
6: Title screen from Animal Crossing (GCN)
7: Title screen from Animal Crossing: New Leaf
8: Loading theme (main theme) from Animal Crossing (GCN)
9: Saving theme from Animal Crossing (GCN)
10: Train ride from Animal Crossing (GCN)
Sorry for the long list. I think that the GameCube games have the best music in my opinion. They were so upbeat and had a jungle-esque vibe that really made you feel like you were in a forest unlike New Leaf where the music is so calm and is nothing like the original games' music (except for 5PM). The movie also had some really good renditions of music from Wild World, especially The Roost which was made longer and more instrumentalised. It's really hard to choose which songs go where because they are all great.
Like a lot of other folks, I'm a big fan of Stale Cupcakes. Also some of K.K. Slider's acoustic versions are really pretty and haunting. I also *adore* the closing music for the emporium. It's a bit melancholy; I love it so much I'll occasionally go in just to listen to it for a bit.
I love the town music when it's snowing out; some of the morning music is too busy (I really don't like the smooth jazz feel of the 8am music especially) but when it's snowing it's much nicer. I really like the night music on the island - so chill. I can fish and beetle hunt for ages to that jam, lol.