Worst thing: God knows best.

Best thing: I've been there and it's really hard. Do you want to talk about it? Is there anything I can do to support you?

Unfortunately, lots of people feel obligated to tell you that "This is in God's plan", "This is for the best", "At least you didn't have a stillbirth", and "You miscarried because there was probably something wrong."

Not a lot of people know what to say, and will fill the silence rather than saying, "I don't know what to say." Sometimes, saying nothing is better than saying something unkind.