Drowning. Jump off a bridge with a weight tied to me, like a cinderblock. Tie the whole works with a chain and two padlocks. One lock to secure the chain to myself, and the second lock to secure the cinderblock to the other end of the chain. Then hop off the bridge.
Drowning is probably atrocious, but this method seems the most failproof to me. Wouldn’t want to use a method where I survived but became severely handicapped. I would trust drowning over blowing my head off with a gun even, unless it was a magnum, and where the fuck am I gonna get that.
I might also consider freezing to death, like lying down naked in snow in -30 weather. But too much time to reconsider and leave, minus most of your digits.