If you have 20k in cash and want to invest in one post-war card...

[Bigdaddy]

So first, I'd narrow the scope down a bit, assuming this is strictly post-war:

• Stick with a main-stream issue for liquidity (demand) reasons
• Go with recognizable names - Mick, Willie, Jackie, Sandy, Hank, Roberto
• For supply sake, I'd focus on cards from the 1950s - early 1960s. Too much risk in the modern, shiny, stuff
• Buy the highest grade you can afford, but beware of altered/trimmed cards. If it doesn't look right, move on.

Given that criteria, I'd narrow it down to a few, again in the higher grades:

1952 Topps Willie Mays
1952 Topps Jackie Robinson
1955 Topps Roberto Clemente
1953 Topps Mickey Mantle
1954 Topps Hank Aaron
1948 Leaf Jackie Robinson

And right now, given the above, I'd go with a '52 Mays.