I am extremely fortunate to have a lot of good ones to choose from. A few favorites, paraphrased:
Friend online: "Ok, so you're being serious, right? Because then that's great! I'm very happy for you and honored that you told me."
(Had a similar response from another friend, made a little trickier because it was on April 1... bad choice of coming out day.)
Young daughter, whispering to my wife: "This means we'll have two girls in the family!!" (Math was off a bit, but the spirit was there. I assume she meant two girl parents.)
Mother: "It doesn't matter, I love you, no matter what." (This is more powerful than it might seem because she had just pulled me out of my seat with a bear hug.)
Very good friend: "I can do the name and pronouns, but I have to warn you, I'm probably going to keep treating you the way I've always treated you." (A more welcome promise has never been made.)