As kid it was getting Hank Aaron's autograph. His being the homerun king was why he was my guy from a very early age.
As an adult, it was actually Hank Aaron again! This time it had less to do with his being the homerun king and more to do with what I understood to be the content of his character after reading numerous books about his life.
I remember Vida Blue signed a ball for me as a kid when he was coaching for the local minor league team. My favorite was getting Mike Piazza's autograph when I played against him. He was my guy growing up.
I have a lot of favorite ones like Jeff bagwell, pujols, trout and Henderson but the one I really want is Maddux! Also I got your letter and once I get a box to send it in, it'll be on the way to your house