Which is more accurate: opk or bbt?


I never got a positive opk reading, but it got to the darkest color before positive and the next two days faded back down. I guessed that I just missed the positive (only tested once a day) but it happened. I have also been taking bbt. A couple days after I think I had a surge my bbt jumped up drastically.

Would you think the opk reading or the bbt reading was more accurate? I’m trying to figure out when my period is due, but it changes based on what information I go by.

Thanks for your help!!