I’ll just post 5 in no particular order if you don’t mind, since I couldn’t pick just three. 
Pikachu MATCHPRINT. Only two copies exist in the world, so I’m happy to own one of those. (And its artwork is also among my favorite for Pikachu.) It really represents what it means to go after all Pikachu cards that exist, and how almost impossible that goal is with cards like these. If I ever get my hands on a 1999 SNAP Photo Contest Pikachu, that one will definitely top this one though.
Moltres BGS-BL10 (pop-1). This Pokémon artwork is my favorite of all existing TCG cards, so to own the POP-1 BGS-BL10 is amazing. Really happy to be the owner of this in my collection.
PSA-10 The Masked Royal. When I browse through my Pokémon binders, my Full Art Supporter collection always makes me smile. Although I love my Pikachu, Seviper, and Moltres collections a lot as well, the FA Supporter collection is probably my favorite and I’m glad I decided to start collecting them. This promo in particular is limited to 100 copies, and it was given to winners of local Japanese tournaments, so actually a semi-trophy. It’s also my most expensive Pokémon card bought thus far, but I’m glad to have this center piece in my (fully completed) Full Art Supporter collection, especially as PSA-10.
IGAS-JP000 Ten Thousand Dragon. Right now I’m mainly focusing on my Yu-Gi-Oh rarity collection. This rarity and card is a very special one. This is the only card with this 10,000 Secret Rare rarity, which was released to celebrate the 10,000th Japanese Yu-Gi-Oh card printed. Because of what it represents for Yu-Gi-Oh in general, and being an unique one-of-a-kind rarity, makes it a center piece in my Yu-Gi-Oh rarity collection.
Carl Hoff’s Double Circle Real 5x5x5. Although I’m currently not actively collecting twisty puzzles anymore, this is without a doubt the puzzle that will forever be the center piece. It was first sold in 2014 when I couldn’t afford it with my Saturday mailman job, but I as able to buy it in 2017. This truly is an engineering master-feat and piece of art among twisty puzzles. In short, it’s basically like solving a 5x5x5 Cube, its inner 3x3x3 Cube, and the inner-most 1x1x1 Cube in a single puzzle, all brought up to the surface. (For those curious, here is a video where I explain it in more detail.)
Greetz,
Quuador