I think it's varied for me between the games. In WW it was fall because I loved collecting the acorns for Cornimer and I loved the mush set. (If there's one set I miss from older games it's the old mush set.) In NL, probably summer because between the dark grass color, the fireworks, the cicadas and the fact I got the game/started the town in midsummer, the day after my birthday, I just have the most nostalgia for that season - though the igloo campsites with the soup pot and Christmasfestive lights were cozy in winter too.
In NH the holidays are gonna look different for people depending on the hemisphere, so I can't really say based on non-nature-based holidays... and I haven't experienced all the seasons myself as a non-TTer. I don't really have a favorite season irl either - if I didn't live in Florida, I might have more of a preference, oof.
But based on what I've seen of the other seasons in-game I think my favorite right now is winter. The way everything is frozen over and the fruits really pop is pretty. The cronch of the frozen grass is nostalgic too, and I'm happy to see snowpeople go back to the way it was before NL - I don't think I ever bothered to make the whole family tbh. Bingo was boring too.
Summer.
Beautiful and green, the way Australia really is. (Even in winter.)
I hope they get rid of the bland depressing snow that has been forced on those of us in the southern hemisphere that don't get snow, and don't want it.
FALL FOREVERRRR <3 I'm totally biased because I really love autumn in general *u* Halloween is my favorite day of the year, I love scary movies and crunchy leaves and costumes and pumpkins and literally everything associated with it. My wedding anniversary is freaking Halloween. I just love everything about the items, the colors, I totally love it. <3
I don't TT, but I've briefly gone to some islands that were in the winter season and I thought it looked really pretty, so I'd have to say winter (without actually experiencing it on my island in-game, lol)