Drowning is the worst death, actually. Anything that involves having your heart stop while you're conscious is also pretty hellish. My heart siezed up on me a couple years ago and more or less killed me for a full minute. Hooray for ventricular fibrilation. Hmm... Best way? Car bomb. Being at ground zero of a massive explosion will kill every cell in your body at once, so no pain. None of that "everything but your head being burned to a crisp" mess.
EDIT: Organ donor... In the US, Ohio at least, when you're getting a learner's permit or a license, you'll be asked if you want to be an organ donor. Simple as that, and they'll put the icon on your card.