Let me start of by saying this is not financial advice but my own opinion. Please do your own research.
I don't invest much in buying cards, preferring to rent more but lately as the prices of some of my favorite cards are going up, I decided to take the plunge. Here are some cards I think will be worth a lot in the future:
Owster Rotwell - The anti magic squad is at your service, besides being a summoner which in itself adds to it's investment potential, it is a great counter against Alric and Delwyn. To the Bronze leaguers tired of Alric, this card is awesome. At its current price, it's a steal.
Brighton Bloom - Another summoner and the best for the flying rule-set. Instead of frantically looking for flying heroes, you can just pick Brighton Bloom and rest easy.
Creeping Ooze (Gold) - The card is spammed a lot but hasn't reached the prices of the furious chicken. Yet.
Spirit Druid Grog - Going out of print soon but not out of style, a great surprise victory card esp for equalizer rule set.
Fire spitter - Solid low mana card.
What are your picks?