I predict that the two dinosaurs would fight to a standstill. Dr. Gregory Erikson, a researcher with UC Berkeley, predicts that the Tyrannosaurus could bite with a force of 3,000 pounds.
? Researchers at the university of Alberta Canada discovered that the Ankylosaurus tail, it's main offensive weapon, had only a limited range, and a Tyrannosaurus Rex, who's capable of 10 to 25 mph, could circle around and keep the tail on the far side. They even began with, "An Ankylosaurus probably couldn’t kill a Tyrannosaurus rex with the club at the end of its tail." Especially if he could never line up a hit.
? ? Walter Coombs' research showed that Ankylosaurus didn't have spikes or spines on it like is commonly believed, so these obviously can't play any part in the "battle." The T-Rex's main skills are a killer offense and evasive defense. The Ankylosaurus has armor to prevent the T-Rex from doling any damage, but can't get any hits itself. Therefore, both dinosaurs (probably the T-Rex first, as it was more intelligent ⁴
⁴) would realize the fight was futile and walk off.