Best OPK?!?!?!?

I need help ASAP! I've been using the first response opk......yesterday I got a very faint line....this morning I used a cheap one as well from the dollar store....the first response one had no line and the dollar store one had a line. I'm going crazy and don't want to miss my chance for IUI.....can some please help me and recommend a good brand!! My clinic said anything but digital due to false positive? ???