House of Nightmares! JUST KIDDING. I'm still recovering from the emotional trauma lol. No but seriously, it's really hard to choose because they were all really fun in different ways, even the House of Nightmares, if only to participate in the collective groaning and bemusement of other forum members.
I found the Celeste's Observatory challenge to be particularly meaningful for me as someone who works in an artistic field but never does that sort of color work. I feel like it forced me to get out of my comfort zone and try new things, and it's inspired me to continue exploring digital painting well after this event has ended, so that is definitely a top contender for me.
I also really liked the Sleepover challenge, because I have found Harv's Island to be a continuous source of entertainment for me; I love creating scenes from my favorite stories or just experimenting with design ideas before I commit to them on my island. Of all of these, I hope to see a Harv's Island competition return to TBT again soon, perhaps in the autumn? A Halloween-themed mini event with a design a haunted house challenge? YES PLEASE.
@Jeremy, I hope you consider this!
Finally, I really enjoyed building my fort for one of the Cosy Photo Challenges, so much that it is STILL up even now. In a weird way, it is a fort for me; the world has been a scary place lately and building such a magical little space felt restorative in a very difficult time. I've been using it to hide and read and it feels like a secret world where I can escape when real life becomes a bit overwhelming. I put a lot of work into it, so I'm not in any hurry to take it down, though I will have to at some point since I'm using pretty much every spare sheet/blanket I own, and plus it's taking up a big part of my room!