Quickest and painless, I'd think shotgun to the head or jumping from the top level of a car park. The pain won't have enough time to really register before you've gone, depending whether one jumps or shoots themself. Jumping onto concrete/street/highway etc. is more preferable to jumping into water, as with water you risk massive injuries plus drowning. No thank you, I'll take a concrete fall - that's a fairly quick "lights out" situation. Guns I have zero experience with however many, many people successfully end their lives this way and again, not enough time for physical pain to hit you as you'd be gone by then.