A quick, painless way in my old days (90-93 year old)
Decapitation, Headshot, Stab to the heart, as long as I don't know its coming, its fine by me :)
The worst ways for me to die would be:
-Drowning (Fuck, about 120 seconds of me feeling my air evading my lungs and water entering instead, while my brain tries to keep me alive, which makes me feel a long long pain and lack of breath until my brain finally gives up and i get unconscious, with my fate handed to the sea)
-Suffocation (Same above, only that its only about 15 seconds and I won't get unconscious, I will die when I pass out)
-Fire (Fuck, about 20 long seconds of me feeling every single part of me being slowly burned off my skin, until it finally reaches some important place, leading me to die in slow-mo with fire on my skin and my pain never ending)
Fuck, if I had one of those 3, I would just be hoping for death instead of trying to get out.