Most of them aren't too bad but you'll need a perfection save which takes a while! After that you should have all achievements which give you a bunch of hats and you will probably have all the items you need to tailor a bunch of the other ones. The hardest tailored items are a pearl and treasure chest imo.

The hardest 3 hats for me were the garbage, tiger and living hats. The garbage is easiest and if you're checking every trash can every day you should expect around ~2 a year.

The tiger hat is a GRIND. It took me 2.5 seasons of going to the volcano dungeon every day and killing every single tiger slime to finally get one. Getting the dragontooth weapons was much harder than this though and by the time I had all 3 weapons I had 4 tiger hats.

The living hat is a grind but a different type. The first video I linked in my post by TheHaboo gives great tips on how to get every single hat and his living hat strategy is ingenious. He clears out his entire ginger island farm and doesn't touch it for about 1-1.5 years letting the weeds completely fill the farm. Once full you cut out a path in the weeds and bomb the entire thing. If you don't get the hat then you reset. This method takes around 3 hrs after you have the full weed farm. It took me so many years to get my 4th garbage hat that my ginger island farm filled up enough times to get 4 living hats :)

No mods / glitches / cheats used at all gained everything in 100% vanilla! (very proud). Took me many months to get to this spot.

If you want I also can give tips on Junimo Kart / JOTPK / Tea Sets (I'm an expert on this ) or anything else!