I tend to play assassin-type characters often, so I can try to give some general advice while attempting to answer your questions.
First, there's no such thing as an instant-death enchantment. You can deal massive damage, which might be very similar, but there's always a chance that you'll run across something you can't kill with just one hit.
That said, there's a number of things you can do to increase those chances to near 100%.
In Morrowind, I'd suggest enchanting your bow with Drain Health + Weakness to Magicka as much as you can afford to. This is a potent combo that can significantly reduce enemy health, and drain health is a particularly cheap enchantment if it only lasts for a second (which is optimal in this case).
There's also a completely unpatched exploit that allows for infinitely powerful potions to be created. With this you can be effectively godlike, which means you can kill anything in one hit, regardless of sneak state. It's kind of cheesy, though.
In Oblivion, you can do something similar with Drain Health, although I tend to prefer using an elemental damage to go with it instead of the magicka weakness. There's a couple of suggestions on this UESP page for bow enchantments as well.
If you don't mind a bit of save scumming, you can pretty easily walk away from a few Oblivion Gates with Sigil Stones capable of enchanting a set of armor with 100% chameleon - this makes you effectively invisible, and all attacks are sneak attacks. (For bonus points, go piss off a guard and watch him run in circles trying to find you...)
In Skyrim, there's not really a terribly overpowered enchanting combo, in my experience. However, you can get some massive damage multipliers for daggers, if you're willing to get in close. Also, with enough Sneak you can hit enemies with arrows, not get a complete kill, but yet not get detected. Finally, there are many abilities and various alchemy recipes that yield a state of invisibility that keeps your sneak attacks flowing.