Magic: The Gathering (M:TG) 2011 core set. It's been years since I've played for realsies, though I recently got Duel of the Planeswalkers on Steam, so I'm not nearly as rusty as I could be.
I got out of M:TG long ago due to the cost of staying up to date with "allowable" cards, the annoyance of mostly playing against decks designed by someone on the internet rather than my opponent, rules disputes, and the constant errata (by which I mean the "official" way a card worked wasn't necessarily what was said on the card). It lost its appeal for me, and competition from other card games were making it difficult to find opponents anyway.
However, I'm done collecting cards. I've got old boxes of them that might be fun to look at occasionally, but I suspect the majority of them will one day be traded into a comic shop for store credit that will inevitably be spent on random trade paperbacks. If I were to play again, I would be happy buying pre-constructed decks that are guaranteed to be tourney legal. Heck, you can buy physical copies of the decks from Duel of the Planeswalkers, though with the latest set coming out, I don't know if those will stay tourney legal (see what I mean about the costs of keeping up with "allowable" cards?).
I'm interested in attending a tournament, because frankly, I need to get out and socialize more, and Magic would theoretically be good common ground and fun. This is, of course, assuming there are other adults at the tournament. I know there are plenty of adults who play M:TG, possibly having played it since it first came out, but that doesn't guarantee they'll be the ones attending the tournaments I have easy access to.
I was actually interested in asking someone at the local card shop about upcoming tournaments and what age group they foresaw attending, but man, it is AWKWARD in there. I genuinely have trouble telling who works there and who is just hanging out, which tells me that not much has changed since high school. I wound up checking out the website to find out about the tourney. Kind of pathetic, I know, but the atmosphere made me feel like an outsider in an unwelcome land, which is kinda weird. One would think a comic book store would be a natural habitat for me.
Although, someone on Twitter was kind enough to link to a Store & Event locator, and some of the locations are colleges. I might want to consider those if I'm hoping for adult opponents. Hell, maybe I can finally find people playing D&D 4.0 with that thing. Everyone I find for D&D the old fashioned way flakes out ^^;