I focus on high grade, low pop. What would be the 5 "grail" cards?!
Would suggest taking a look at the following: Allen & Ginter N28 John Sullivan from 1888. There tends to be one or two that move in every PWCC pre-war auction, though the high grade examples have stayed pretty quiet until recently. 1911 Turkey Red (T9) Jack Johnson. Super low pop, but a fantastic cabinet card. They don't come up for sale often, definitely the hardest to get your hands on. 1933 Sport Kings (Goudey) Max Baer. These come up often enough, even some raws available regularly. Absolutely love this one. T218 Jack Johnson - Pretty popular boxing set, so the Johnsons get high prices 1948 Leaf Joe Louis - This whole set is pretty fantastic if you like Leaf's super bold color approach. Joe has a higher pop, but that's relative – only 348 graded by PSA!