Games that can be played with just normal playing cards that are engrossing enough for a hardcore gamer
I generally enjoy trick-taking games as they involve a bit more strategy than "pick a card that's the same color".  Examples include Hearts, Spades, Pinochle, Oh Hell, etc.  

The interesting elements to me are keeping track of what cards have been played, what order to play your cards in to achieve your goal, and trying to figure out your opponents' strategy.