I'll probably be embarrassed by these answers when I hear the right answers
but I'll take a stab ...
The house's only edge at blackjack is that if both the player and the dealer bust, the house wins. Every other rule is in the player's favor.
Therefore, I think the card that is worse for the player if the dealer has it is the 7. It induces the player to hit, and perhaps bust, with the least chance of the dealer's actually being pat. He/she might have busted, had the player stayed.
The other two cards are probably 8 and 9.