James Cameron has never shied from challenges. And, some would say, potential folly. After all, who starts working on a film — like he did with Avatar — before all the technology to create it exists? Avatar stands alone as a stunning achievement, but one senses that Cameron set out to best some specific effects standard-bearers, from lifelike CG characters to digital stampedes. Do Avatar’s set pieces eclipse these earlier triumphs? In some cases, the answer is a resounding “yes” — other times, it is a progress-rebuffing “no.” We pick the winners.