Sublethal dose of morphine, followed by a 10x lethal dose. That’s what my doctor said is the best way to go.
Firing squad, no blindfold. I want them to look me in the eyes as they pull the trigger.
Bastards.
This was actually a choice, though it was more jokingly discussed. Basically, they had the ability to slow or even stop aging, but they also had the ability to speed it up very quickly, so you could be killed this way, at least in theory. Not sure how fun it would be though.
[QUOTE=zweisamkeit]
Are my choices limited to those in your OP, or can we request a different method?
[/QUOTE]
Anything you can imagine would be possible in a science fiction scenario. Personally, I’m leaning towards running coach’s antimatter, though obviously they would put you in an escape pod and shoot you out to a safe distance before letting you open the antimatter container. But it would be VERY fast, at least.
I had a good laugh thinking about someone being hung in space. They put the noose around his neck and kick the stool out from under him… Three days later he’s still alive, floating there and the guy who hung him screaming “Why isn’t this working!”
Would hope they give me the ‘honorable’ option of the 1-bullet pistol. Headshot.
Otherwise (if I have alternative options), the Guillotine. Hey, they ought to have it pretty well perfected by then and can’t think of anything much surer and quicker–snip and it’s off.
They have plasma rifles, so the headshot would effectively be a guillotine, since one shot and your head would be gone. 
Strap me to a missile and fire me at an enemy.
In case anyone cares, the book is:
[spoiler] Warspite, book 4 of the Ark Royal series by Christopher Nuttall
[/spoiler]
Bullseye me into a black hole.
Taking my accuser with me.
Guillotine. I want to be conscious, but I want it to be quick.
Let’s see:
First choice: old age (seconding Tim R. Mortiss).
Second choice: snu-snu, now that I’ve looked it up (thanks, susan).
If money’s no object, my third choice would be to be left in a very large and well-insulated or artificially-warmed airtight dome on the surface of Mars, with enough food and water to last me until the oxygen ran out.
If we had to stick to traditional means of execution, I’d choose the guillotine. It would be nearly instantaneous, and I’d expect brain death from lack of blood circulation would occur before the shock wore off and pain could set in.
Yeah, I think this might be good as well, though not sure if death, when it finally happened subjectively would be effectively instantaneous (would you die from the massive radiation before you are spaghettified or would the gravitational tidal forces rip you apart at some subjective time after you cross the event horizon?)
If they insist on doing things now, then firing squad. Just let me stand over in front of this window.
Picking last meals is way more fun, but lethal injection, I suppose.
ETA: What’s the “Q” stand for? 
Harry Mudd on Star Trek faced this very issue. Apparently he escaped, though, and wound up ruler of a planet of androids, in “I, Mudd.” Considering his punishment at the end of the episode, he may have wound up nagged to death: “Harcourt! Harcourt Fenton Mudd! You’ve been drinking again…”
I was gonna type: just shoot me. But in space I guess that wouldn’t work too well. Hmm. Not a whole lot of great options. Maybe just being shot into space. Relatively quick and relatively painless (I guess).
Hanged.
Then the joke doesn’t work.
Instant vaporization.