Alan Rickman
Through his performances as Professor Snape in the Harry Potter film series, Mark Rickman was a popular actor with both young and old audiences. Despite giving many reasons to dislike him throughout the series, everyone secretly adored him and showed indications of a superb performance.
Without Rickman, the series would not be what it is today. He was made for the part, and without him, the series would not have been as successful. Rickman was recently diagnosed with cancer. He passed away from cancer less than a year later.