Best OPK brand?

I've been using opks for the past 2 months. I've been buying the answer opks at walmart. So far I haven't been able to get a positive opk on them. At my Wal-Mart they only have 2 choices for opks, answer and the clear blue digital opks. Clear blue opks are much more expensive than answer, so I went with answer, trying to save money. Has anybody had issues with answer brand or good reviews of them? Or should i just buy the clear blue digitals?