Hmm, every time I think of an idol that I think is least talented, I think "well, but they're good at 'this' though". Taecyon is, um, not exactly a good rapper, but he's an incredible model. Nickhun or Siwon maybe, but I actually like their mild, imperfect voices and would be sad if their derpy faces and personalities (and beautiful bods) weren't in kpop land. Goo Hara, but she's not a half bad dancer and a lot of people love her face and personality (and yeah, I include looks and at least a personality to back it up as talents. It's part of entertainment). HyunA, but I kind of adore her when she's on variety shows and whatnot. Face and personality, again. I've seen a lot of lack of talent this year with the avalanche of debuts, but I think it's a bit unfair to include them. TLDR to say, maybe I don't have a least talented?
As for most, the Kpop idols who have impressed the most consistently would be, with just thinking about it now, IU, Sunny Hill, Sistar, Brown Eyed Girls, 2AM, Taeyang, Tablo, Kyuhyun, Ali, and CNBlue (indeed, let us wage war Acting >:O ). And see, right there, I love CNBlue and think they're very talented while Acting listed them as not so much. Music is rather complicated because there's so much perspective and taste and different strokes, different folks. Even with something as seemingly concrete as measuring talent in certain areas.