Evolutions.
One, it got me into the game. But two, it's kind of Wizard's fault.
If you've never played Magic, you might not know that Wizards have an exact opposite approach than TPCi's "REMEMBER KANTO?". Yes, they still often pander to nostalgia (I mean, it's a 26-yo brand), but they equally hate and reject their old sets and design philosophies. They've openly stated that they won't bring back "old card frames", even as promotional cards, because they subjectively don't like how they look.
In that case, Evolutions is everything I wanted from Magic, but in Pokemon. A massive nostalgia kick that goes all out and isn't pretentious about it. I just wish it was bigger and contained casually-playable Supporter cards.