Well, for me, the "worst" is definitely Mass Effect 3.
It's not the "worst" in terms of everything in the game being worst of the whole year's worth of my own gaming time. It's the "worst" because ultimately it's the most disappointing. It's not only the most disappointing of 2012 but, for me, the most disappointing game of the past couple of years, on so many levels.
There are some aspects of the game I liked, to some degree, such as the combat system (only because it actually fits the shooter that ME3 actually is, rather than being a necessary combat system as is) and the "new" Alliance Normandy itself (never really liked Cerberus' SR-2 in ME2, never felt "comfortable", nor "mine"). I also liked some missions, such as the Genophage/Tuchanka arc, or the Geth Vs. Quarians/Rannoch arc, along with the Grissom Academy mission (nice changes for Jack, a bit grown up, more mature and responsible, finding purpose in training biotic students and giving them purpose too, I liked that). Additionally I did appreciate some of the short "cameos" such as the obvious Conrad Verner, or the guy from the ME2's Overlord DLC (at Grissom Academy I think it was). And I kinda liked the overall Normandy crew's banter between each others prior or after most missions (although I didn't like the fact that most of the time it was just Kasumi and Zaeed ME2 DLC click-on-character-for-some-preset-dialog style conversations that weren't meant for wheel-style interactions and investigations in character's own back stories or current status discussions).
I'll never deny that ME3 has its good (not many) and "decent" moments, but ultimately, to me, it definitely is the "weakest" title developed by BioWare since Jade Empire (I really tried to like Jade Empire but... sorry... can't, but that's another topic). But beyond that and how overall the game not only feels but was obviously rushed out of the door (despite that one delay of a few months, which should have apparently been a year or more instead) the whole ending mess (Extended Cut or not, doesn't matter either way) was the nail on the coffin. But regardless of the campaign and because the combat system alone is definitely the only really good aspect of the game I managed to "get into" and over time actually liked its multi-player aspect more and more. Which, to me, is the proof that ME3 was taken on the wrong "shooty" path, since despite the fact that I do like ME3's MP I ironically believe that it doesn't have its place within a franchise like Mass Effect, which should have remained purely single-player and story-driven (and that's regardless of the supposed fact that they had two separate teams during development so that both would focus on their respective MP/SP aspects of the development process).
Also, what doesn't "help" me come up with other "worst of 2012" choices is simply because ultimately last year I only bought five games total, the rest of which I own happen to be Free-to-Play or Humble Bundle style or gifted Indie games (and out of the five games I actually bought two of them are old school games from GoG.com). The only "big name" games I bought last year were Mass Effect 3, Diablo III and Guild Wars 2. And, yes, even though I don't play Diablo III much lately I actually enjoyed it enough to let it pass my own 2012 Ban Hammer judgement only to focus my wrath towards ME3, which really deserves it. Also because DIII doesn't take itself that seriously, it's just a grind and click fest just as much as D2 was, and I enjoyed D2 for nearly seven years of my life, so I wouldn't want to be to inconsistent to that level and just blindly claim that I "hated" DIII "just because". I do really dislike the whole RMAH idea but overall I can ignore it and just play on my own. Even if after a while it gets boring, but so was D2 getting boring after a while, which itself doesn't make a game bad anyway, it's normal to get bored "after a while" on many things in life but you still enjoy them when you do take some moderated time to play them for the heck of it.
So... yep, ME3 it is for me.