Not baseball, but probably an SGC 88 '48 Bowman George Mikan. Amazingly my dad let me borrow money to buy it for $5k, and did a quick flip for $6k back in like 2002.
I also at one time had a PSA 10 '86 Topps Steve Young rc (pop 3 back in 2002 and probably always will be). I should have held - that could easily sell for $3K-$4K now.
Heyward Super (which I still have) is up there too of course...he better be healthy, dammit.
*Looking for rare/low numbered Randy Moss rookies (IE E-X Credentials, Skybox Rubies, PMGs, SP Gold Auto /18, etc)
*Also looking for 1997 PMG Red football for set