Looking through a site with all competitions ever held, I noticed that every year, hats are different. Is there any basic hats that don't change? I just wanted to write(not a question, just for me) how to earn every hat, but now I hesitate.
Yes, they used to be totally different every year until last year.
Last year, it was Winter Bash's 6th anniversary so some hats were reused:
But not just any hats: in celebration of the 6th anniversary of Winter Bash¹, some of the most beloved hats from years past will make their triumphant returns to the Winter Bash stage for a repeat performance. We’ve switched up the triggers to ensure hours of hat-gatherin’ fun. (And if you poke around, you might find a few newcomers waiting in the wings to be discovered, too!)
And on last year's hat post, it's noted which were the hats reused.
I guess they continued the tradition this year and a ton of past hats was again reused.
There are also new hats, some of which are below.
The set of hats changes every year. That's part of the fun -- to see what new ideas they come up with each time. This year, some of the new hats were based on suggestions from the community in a contest held last year. (My contest entry led to one of them.)
Some hats or hat triggers do repeat, not necessarily every year. Winter Bash: Common Hats lists these repeated hats.
Each year there is a post describing how to earn that year's hats. See, for example, Winter Bash 2018 Hat list. If you want to compile this information for yourself, look up these posts for each year (if "hat list" doesn't find them all, try the tags).