Supposedly, Ye Eun has the best english (studies it very hard~ So there's rarely any mistakes), then Sun Ye (grammar inconsistencies here and there) then Yoobin (She forgot so much xDD There were times in the English MCs where she forgot the script, so I assumed she actually went and memorized it instead of understanding it) and Sun Mi + So Hee seem to also know english very well, but they don't talk enough for me to accurately judge. For Sun Mi though, I think there was one point where she jokes with Ye Eun in complete english so I guess she knows it pretty decent. xDDD
Overall:
Ye Eun = almost like a Local-speaker
Sun Ye = Obviously more than the stereotyped Asian restaurant worker
Yoobin = BoA (back then)
Sun Mi = Unknown
So Hee = Unknown
There's the WG english scale o_o;
Info based on their english chusseok interview, interview at that party thing, and their Welcome to Wonderland eps (ep 2 ftw!) xD
Maybe after their what, three month stay in NY, they'll speak it better than before xDDD