Clearplan was the most accurate, but I got a negative on one the day my daughter was conceived (I actually never got a positive OPK that month!) I'm not a big believer in them. I've read that just BDing daily when the woman has fertile CM has the highest rate of success, something like 60% per cycle.
ETA: Answer OPKs didn't work at all for me. I got positives every time I tried one.