Best test? And when to test?

TTC #3 for 15 months now. Last Monday I got the peak on opk. AF is due 5/27. I want to test now but I know it's way too early. I just can't help it, esp b/c my sister just found out she's expecting. When should I test and which brand is the best for early testing? I don't want to wait until I'm late to test, I'm too impatient.