Anne Rice. She sparked my love of vampires, and I regret not contacting her when she was alive to tell her that. I'd love to be able to tell her in person how much her writing inspired my own vampires.
Diana Gabaldon. I absolutely love her Outlander series. I'd love to pick her brain about writing and researching and tell her how much I love her writing style.
Hmmm... those two were easy, but picking a third seems a little more difficult...
Queen Elizabeth II. The more I read about her, the more I would have liked to meet her in person. She was a part of so much history and I can just imagine what she could share!
I think every writer experiences something along the lines of "Writer's Block" at some time. There's always those times when our brains just can't put words on paper, and the more you try to force them, the more difficult they seem.
When it happens to me, I step back from the current story I'm working on and maybe work on another story, another creative pursuit, or just read.