OPK help needed!!!

Hello ladies,
I am ttc for 4 months but no luck..Every month I get disappointed when af shows up.
 I would like to use Opk test from this month but not sure which one to buy.. Will you plz help? Costly ones like clear blue works best or I can get amazon cheapies, 30-40 tests for 20 bucks...
Please suggest.. I dont mind spending money as long as I get accurate results..
Thanks in advance.