I could tell you but I'd have to kill you afterwards
There's lots to choose from...Whether it is rare overseas type cards or obscure prewar American regional issues. IMO you really can't go wrong with scarce cards of all-time greats of the sport in career-contemporary issues. I don't predict values but some of the cards I enjoy having are cards like these:
I figure if I enjoy them others will too and that will work to my advantage in the future if I ever decide to sell my collection. I'd say learn what you can and then trust your eye.
I also consider good photos to be a great deal right now.
In the future collectors are going to look back on the era of the great newspaper archive liquidations the way collectors who started in the last ten years look at the late 1970s for baseball cards.