Great topic!
Oh boy....I've collected a ton of different things over the years, so I'll try to make this brief
....
....I guess my biggest collection is currently my stuffed animals/plushies-branching out from there are my sub-collections (is that a word?) of Beanie Babies, Itty Bittys (little Hallmark plushies) and Disney Tsum Tsums. I'm in my 20s and am not embarrassed about how much I still love them. They've just always made me feel happy, relaxed and safe, and that's never changed. I carry a mini Tsum of Dory in my pocket everywhere.
.....my second-biggest collection is probably my "hard" or plastic/PVC/Schleich/Safari LTD/Happy Meal/surprise bag/vinyl toys. A lot of them I kept from when I was a kid, and I used to score bunches of them (ones from the 80s and 90s mostly) when I'd go to flea markets with my dad years ago. I also collected My Little Ponies (the 80s ones are/were my favorites, although I do like G4) and I'm currently into the Smurfs. My favorite Smurf from the cartoon/movies is Clumsy, but my favorite Smurf figurine is probably Poet Smurf from 1972...every time I see him with his tiny quill pen and scroll of parchment, he reminds me I need to get back into writing again.
I tend not to be too open about actually liking Smurfs tho...my mom has always *hated* them for reasons she's never really explained and that I don't quite understand.
I also have a small group of the toy Snoopys that used to be attached to the little boxes of Whitman's chocolates.
I've also happened to amass a small amount of Troll dolls, too...my fondness for them was rekindled when I saw the Dreamworks movie.
.....and then there's my collection of fidget/stim toys. It took me awhile to find just the right kinds of stim toys that worked for me and gave me the right kind of sensory input/distraction from BFRBs. My favorite stim toys in my collection are probably Tangle toys, smooth stones/crystals, a tiny fidget cube, a spinner ring, a Calico Critters cat, and of course, any tiny stuffed animals that are soft and fuzzy.
......Die cast cars! Mostly of the anthropomorphic characters from Disney and Pixar's "Cars" and "Planes" movies. I was going through a really hard time when the first Cars movie came out, and my collection of die cast Cars characters were my only friends and confidants during that time and also even after I was in a safer place. They've been my biggest special interest/passion for a good chunk of my life, and with Cars 3 being released this summer, it'll soon happen all over again!
Well, those are probably my biggest collections to date, and the ones that bring me the most joy. Buttons, pins and key chains are also a favorite.
I also still have a small deck of Pokemon cards from back when Pokemon became a hit in the US...I was about 6 or 7 then. I didn't have a whole lot of Pokemon stuff - just a couple of books, said deck of cards, and some plushies (my favorites being a Jigglypuff keychain, a Togepi bean bag from Burger King, and a Pikachu plush my aunt gave me). I also went to see a couple of the movies. As far as the cards went, I was mostly drawn to the art styles on each card...I didn't really collect any particular type. I knew kids at school had entire binders full of their card collections, but I only ever had enough cards to fill a small Walgreens photo album. I was content with my small deck of cards, tho.
One of my favorite Pokemon cards was a holo card of Entei that they were giving away at theaters when the Pokemon movie with the "Unowns" was released.