New Years definitely. The snow, the fireworks, the countdown, the villagers hanging round the plaza with their hats, and the music! It's not that interactive but it is beautiful.
My favourite event is Toy day, you get to dress up and give the villagers what they ask for. Although, last year I failed badly, and I just guessed them all
Halloween only because of the awesome furniture you get. All the events are like a chore and it takes a lot of time to do them. However I like the prizes you get xD.
Toy Day or New Year, I couldn't decide honestly!! New Year'S countdown is amazing and giving gifts on Toy Day and making your villagers happy is also very nice