I like some of everything I guess. I try to look for a few things: iconic images, images that I consider to capture the essence of the subject, images from important events, and early career images. Here are some examples:
Iconic: 1920s Apeda studio Jack Dempsey, used in many cards, premiums, etc.:
Iconic: the essence of the Cincinnati Cobra, Ezzard Charles:
Iconic: the Brown Bomber, Joe Louis:
Important event: Louis, training for the Braddock fight:
Jim Jeffries resting at the camp training for the Johnson fight, 1910 Dana Studio stamped Type I:
Early career Clay/Ali:
Early career Gene Tunney:
David [Skywalker] Thompson in college:
ETA: plus, it has to have a good composition, decent focus, not to much damage or editorial markings [though I will buy anything with these defects if the price is right]. If I happen to collect a subject, I will accept more issues. I also will grab photos shot by 'major' photographers if I can find them at what I think is a good investment price:
1911 Fred Welsh by George Grantham Bain:
1967 Cazzie Russell-Rudy LaRusso by George Kalinsky:
And then there's just plain fun stuff:
