Most pre 1970's fighters had a lot of it, recent ones are Tyson, and Duran. Dempsey had great head movement. Many of those old timers, 1920's and 30's, held hands low and relied on head movement so much.
I'd say Whitaker and Benitez take top honors. Fluid, natural, well schooled, and in total control. They took evasion to an art form. Mike Tyson had incredible movement from a technical standpoint, but he still got hit a lot. Most of that had to do with patterns to his angles and bouts of laziness, though. I'd still show clips of him to teach a kid how to get inside.
Jersey Joe Walcott- i wouldn't say the best at all but he's mainly remember for loosing to Rocky and Louis, he would use very subtle head movements to move out of the way and be in a position to punch back, not saying the best just should be recognized imo.
Whitaker, plus he was a south paw so it gave his style an advantage that allowed him to bend in an unnatural way