Magic: The Gathering
I played and collected quite a bit when it was new / I was a teenager. Even had quite a few beta cards / black lotus back in the day but the game really hit it's stride with 4th edition / Ice Age. After that, I kinda started realizing that the game was fun but it was a giant money sink. Sold all my cards.
A few years ago, my (at the time) teenage kids were really into Magic: The Gathering. So I started buying up the new sets, collecting all the cards, building awesome decks using the new mechanics, and teaching the kids how to play. Fundamentals of strategic deck building, how cards don't have to be rare to be useful, mana distribution, mana cost ratio spreads, the works. We would build decks and play against each other all the time, it was a lot of fun. This was right after Amonkhet came out, and I collected all the cards / sets from that point forward up until the newest Ravnica stuff. So for about two years. My kids more or less lost interest and so did I.
Now I've got all the cards from all of those sets just... sitting on my shelf in neat little themed soft binders, everything in order. Not sure if I'll sell them, or just hold on to them.