Guy Lafleur is the most well known name that comes to mind here. Wore a helmet up until 1974. Starting in the 1974-'75 season he took it off and that was his breakout year. It has often been said that once Lafleur took the helmet off he became more free. But that is more figuratively speaking since we always associate him with the long wavy hair blowing in the wind.
A lot of guys were forced to use them when theyd suffered injuries, but just temporarily including Gordie Howe in 1962... Red Kelly was another. He didnt wear a helmet every game despite being a major advocate from early on to have them mandated, and was in fact instrumental in making that happen in Ontario around 62/63 at the amateur levels.