I've very little experience with asphyxia, but I think I can add some info.

If someone sneaks up on you and wraps a cord around your neck, the first thing that's going to happen is your reaction(obviously). Most likely, your reaction is going to be to immediately grab for whatever's around your neck and try to get it away. Simultaneously, you're going to shift around your center of gravity, trying to throw off whoever is behind you. You'll thrash to the left, right, pull forward then quickly backwards, anything to shake the person behind you and force them off of you. While grabbing at your neck, you'll probably cause cuts and gashes with your fingernails.

First thing you'll feel is panic. You're in an unknown situation with an unknown person behind you. If you're really interested, you could read up on adrenaline and what it does to people. After the initial reaction of panic you'll start to feel the pain. The first pain you're likely to feel is the cord digging into your flesh. That's not going to feel good. The second pain(which will probably be ignored) would be your fingernails scraping at your neck trying to find purchase. Third would be the pain of not having breath. This will come last, but it'll be the strongest. Your brain will go into overdrive, doing anything and everything to escape and you'll likely be hyper aware of everything.

I don't know about the ending, someone else will have to help. I've heard people describe a sensation of floating away, slowly losing the energy to fight back. I would suspect there would be a peak of excitement, pain, and fear then it would gradually all go away as your brain is starved of oxygen and you're unable to continue the fight.

Many people every year die of auto-erotic asphyxiation(strangling themselves while masturbating). I think that's the greatest testament to the fact that lack of oxygen would magnify the feeling of orgasm. I can't imagine people would accidentally kill themselves chasing an extreme orgasm if there wasn't something there.

As to if a person can orgasm while in pain and/or distress, the answer is yes. Many women have reported feeling an orgasm while being viciously raped, for example. It is physically possible to have an orgasm against your consent, however, it's not terribly likely. If you were writing a story based upon the odds, I'd say the odds are against having an orgasm while being strangled to death(specifically if the whole situation isn't consensual). However, given that you're writing a story, go nuts. As to if it would be the 'best', I don't know if you'd really be in the right state of mind to enjoy it if you're currently being murdered(unless that was what you wanted).

Hope that helped.