I am most challenged by playing cash games against the world's top players. These games force me to think several moves in advance, like in a game of chess. And though I also find tournaments fun to play, they just don't provide the constant brain buzz that cash game players crave.
When you play No Limit Hold'em, the ideal strategy is to take minimal risk, do little bluffing, and hope that weaker players call you when you have a strong hand. But that's the perfect world. Sometimes you'll face opponents that play very conservatively and will rarely pay you off when you have the goods.