I think the toughest modern guys at the moment are Michael Jordan, Tom Brady, and Tiger Woods.
Brady does do private signings but it's usually something crazy like $1,500 and up. But I think he's probably next to impossible under any other circumstances.
I don't think Jordan or Woods will ever hit the show market. They both did private UDA signings in the past but I think those days are probably never coming around again. Highly doubt you'd ever get them in person either.
Then there are guys I know just don't like signing. Ken Griffey Jr is one. Zach Greinke another. I think Barry Bonds does do private signings once in a while.
Here's a cranky Greinke:
