Let me start off my saying that this is all based on observation not fact. If you have any questions leave a comment. There is no show that tests their English and see how well it is and its no secret that they are taught what to say on shows or interviews, so bear with me.
Ok, now the only ones left that actually speak English pretty good are "S trio".
Sunny-she is confidence when she speaks English,has a slight accent when she speaks, reads English novels
earliest found English:
link
Sooyoung- She needs more confidence when she speak English, has a slight accent when she speaks, has an English speaking roommate
earliest English:
link
Seohyun- Se has confidence when she speaks English, has a slight accent when she speaks, studies English
earliest English:
link
I personally think Seohyun Sunny and Sooyoung rank after Tiffany and Jessica.
Latest English from all of them:
link