Alexander Cartwright isn't common but isn't that difficult. There is one right now in REA. Josh Gibson is more rare, but even his signature comes up once in a while.
I would say the toughest are (and this can be found in numerous threads) in no order:
George Davis
Frank Grant (no known examples exist)
Pete Hill (see above)
Cristobal Torriente
Sol White
Jose Mendez
Bob Feller
