In blackjack, the cards that are better for the player are the high cards, specifically the 10, J, Q, K, and A. This is because these cards increase the probability of getting a blackjack (an Ace and a 10-value card), which pays out at a higher rate.
A key difference between Blackjack and Spanish 21 is that all 10s are removed from the deck in Spanish 21. A Blackjack occurs when the first two cards that are dealt equal 21 a combination of a face card (Jack, Queen, or King) and an Ace (which can have a value of 1 or 11, depending on your hand).