I guess I'll have to say The Spoils. I've spent so many years playing it. When I started playing it was a funny version of MTG, which I hadn't really played in many years. It drew me in with it's quirkiness and the promise of a great cruise, but made me stay due to the excellent gameplay. I really haven't played any other card game where the mechanics felt so crisp and precise.
Another game that I've been enjoying is Ashes: Rise of the Phoenixborn. It's very much a resource management game. Where you have cards, actions, dice and life as resources. Despite all "random mechanics" in the game, it's surprisingly non-random. You get to choose your starting hand and you can turn your dice to face the side you want. You also have a semi mulligan each round. The game looks really great too. There was a single artist who did all the artwork, but they recently got a new one to do it instead, since the old one wanted to pursue other stuff. But it's still very coherent. That is a plus in my book, but the main pull for me is the mechanics that are surprisingly heavy. It's also an LCG model, which I really like.
I think I would have to mention Hearthstone as well. While I have some stuff that I'm not super fond of (mainly randomness and cost/f2p model), it's still a pretty good casual mobile game that can scratch my card game itch while on the subway.
I've also been enjoying MTG Arena lately. While MTG is not my ideal game, it's still fun as a f2p game. I still don't feel it's worth it to invest in it IRL or to pay for digital packs.