Need a bit more definition as to the question.
Top 5 most expensive?
Top 5 most important for a collection?
Top 5 rarities?
This is pretty much the most expensive list:
1948 Leaf Graziano is far and away the most costly
E125 Johnson is probably #2
T226 Johnson is probably #3
T229 Kopec Johnson I'd rank just below T226
For the final spot I could see any of these:
T229 Pet Johnson
T220 Silver Border Jas. J. Corbett or Mike Donovan
1938 NX5 American Sweets Joe Louis
They rarely come to market, so the issue is really academic until we get a few on the ol' auction block.
Now, if we go with essential for a collection, I'd argue these:
T218 Jack Johnson side view: the key card to the most widely collected T set
1951 Ringside Marciano: the key RC in Topps' only boxing issue
1960 Hemmett Cassius Clay: IMO the RC of the most important boxer of the latter half of the century.
1935 Pattriouex Joe Louis: The RC of the most important boxer of the first half of the century
1997 Brown's Mayweather or 1999 World Boxing Pacquiao or 1986 Panini Tyson: the most important regular issue cards made in the last 30+ years.
But if you are looking for rarity, there are just a ton of possibilities, from 1 or two known example type cards to SPs from major sets. I honestly don't know where even to begin such a list. 1948 Tabay Joe Louis, issued for a month in Spain? 1931 Bigott? Maybe a nice Trinidad y Hnos. Dempsey premium? Or a Lucky Heart Louis? How about the 1904 Sporting Life Jeffries-Munroe cabinet card? 1910 New England Gum Co. Jeffries and Johnson flip card? I don't have an answer...
Perhaps a better frame on the question is to ask which 5 cards you would most want to own if you could not sell them, making value irrelevant. My list of
favorite cards to own would be these:
