I use the clearblue digital opk. I love it because it's foolproof. You get a smiley face when it's positive and you don't have to deal with trying to decipher the lines. Good luck!
My cycles normally are around 30 days...but for the past 3 cycles, I have been off due to not ovulating. I don't really know what day I normally ovulate as since Ive been testing, I haven't O!! I just didn't know if maybe I was getting a false postive or something.
As for the CBFM....that thing makes me so angry! I have never got anything but a LOW on the thing! And the month that I do O, the stupid thing isn't having me test.
How long are your cycles? What day do you usually O? I don't use a particular brand as I only used them for one month while on Clomid and got a positive. Before that I used the clearblue fertility monitor which never predicted even a high reading for me. I can't remember the brand I used the month I got my BFP they were so cheap that I bought two boxes.