I believe that if I had to choose a cancer to kill me, it would be brain cancer. The brain has no nerves to detect pain, so any pain felt as the cancer grows would be to adjacent tissue that feels the swelling.
Still, it can't be fun.
My father had brain cancer (1961 - 1962). It affected the speech center. For the last six months he could neither speak nor write. We never had a chance to say goodbye to him in a proper manner.