which dinosaur has the biggest brain
This depends on what you mean. If it's the biggest brain of any dinosaur period, then T. rex's five-foot-long head houses the largest brain: around the size of a human's. Of course, this isn't saying to much: we have a three pound brain for a five-foot, 200 pound body whilst T. rex has a three pound brain for a 45-foot, five-ton body.