What opk has given you best preg results?

Annie • Married nearly 5 years. Mama to a sweet boy and many 🌈's, Ttc baby #2
Considering getting an opk. But what brand is best, but doesn't cost a fortune. And where can I buy it?  Also how do they work? I've never used one so don't know when you need to start using it in the cycle... Ttc baby #1