I haven't played them all yet but I have voted for Festivals, Bunny Day and Harvest Festival.
Festivale is really fun, catching all the feathers in the confetti and trading them in. When I played it this year, I got something like 30 pieces of furniture haha. I look forward to seeing it again next year!
Bunny Day / Easter was also really fun because there were so many different ways to get eggs and they all had different designs which was really awesome.
I haven't played this but I'm really looking forward to the Harvest Festival this year. It looks like a great deal of fun and also cool food items are great. Buying harvest furnitjrs that might fit in with the Pave furniture through November sounds really fun too.
Some of these holidays I never partook in (either because it's another country or because I do my acnl hiatus during that time), I've always enjoyed the fireworks festival! You can get cute prizes, light up fireworks, and watch fireworks with your villagers.
Harvest Festival, Halloween, and Toy Day are my favorites. Harvest Festival is just fun because you can spend all November getting ready for it. Plus Franklin's little dance when you are about to cook is pretty cute.
Halloween is fun overall because I like trying to scare my villagers and then try to find Jack. Hours of entertainment right there.
And then Toy Day is the best because of the music, the town looks really pretty in the snow and the lights on the trees. I don't like my villagers replacing good furniture with their presents, but oh well.
I'll also add Valentine's Day in there too. I like the presents you can get during it, and the cafe serving hot chocolate is a nice touch as well.
I voted for a bunch, but I recently got to play the Festivale and I just have to say I love Pave. It's really fun catching the feathers and playing the games (though I don't like the rock-paper-scissors game much). I love giving the feathers and watching Pave do a different kind of dance for each color, especially the one that he does for the rainbow feather.
I also love Toy Day and making my villagers happy with their presents. It's also fun scaring the villagers in doors for Halloween (especially the crankies and peppies). I haven't experienced Harvest Day and April Fool's, but the descriptions make them seem like really interactive events. I can't wait to experience them sometime!
Halloween is definitely my favorite event in the game. I love having a bunch of villagers wandering around trying to earn candy. I love going into the others' houses and scaring them. I love both the Creepy and Spooky furniture sets. The pumpkin masks are awesome! It is also one of my favorite days of the year in real life as well.
Behind that is Toy Day/Christmas. It's pretty fun to be able to go around and give your villagers presents that they've been hoping for.
I've only partaken in one Harvest Festival, and while I enjoyed it, it seemed sort of confusing. Hopefully I can figure it out the next time it swings around in my town.