I think they might be the best known K-Pop group in the US, even my friend who is biracial (black and white, NOT ASIAN) knows Girls' Generation's song PARTY somehow, and she doesn't stan or know pretty much anything about K-Pop.
EXO is also kind of big in the US, but not as big as SNSD. I say this because of all of the American tumblr EXO stans (

) and because the Monster music video went number one on the US iTunes video charts.