If we're looking solely at commercial K-pop music over the past couple of years, the best release would probably have to be f(x)'s Pink Tape. There's been a bunch of other excellent albums of course, but Pink Tape received critical acclaim in both the East and the West and is generally considered by idol fans as the current standard when it comes to albums from glossy girl groups.
SM Entertainment has more hits than misses, but when they get it wrong they really fuck it up. They've had a few major blunders over the past year or so, including Super Junior and Super Junior-M's atrocious comebacks, Henry's unlistenable solo career, and TaeTiSeo's life-alteringly disappointing "Holler."
It's taken me a while to do a post on f(x)'sRed Light comeback because I've been trying to decide whether I actually like it or not. Much of that has to do with what f(x) is as a group, and the way that they've been designed by the SM laboratory. Both their image and music are experimental, but in a mainstream way; their fashion is cool and cutting-edge, but still wearable, while their music rejects the typically uncool sounds of 'cute' k-pop for a style that I'd describe as a slightly more avant-garde version of Britney Spears. Because (for some reason) most K-pop listeners consider trendy fashion and electropop to have more credibility and authenticity than matching stage outfits and Asian-y bubblegum pop, it's easy to fall into the trap of just praising f(x) for being f(x) without really thinking about whether or not they're actually any good -- or at least if they're really that much better than other Korean girl groups.