My dad passed away while he slept taking a nap. The way I think is the most best way. What's the next most best, least painful?

Next best - possibly a sudden, massive heart-attack; but though it  might be quick it is not peaceful or pain-free.

As it happens I read an article recently in a science magazine about dying, written by a nurse with over thirty years' palliative care experience.

From what she wrote, your Dad's peaceful passing is the way most people go: a series of deep and shallower sleeps, with no pain when awake, until the end comes in shallow sleep. She added that this can be so gentle it is common for those watching to not realise for a while that it has happened.

The article mentioned the rattling noise sometimes heard in the breathing, as simply saliva bubbling in the throat. It is not a sign of distress.  Similarly with moaning sometimes heard: that happens only because the vocal chords have relaxed, not a sign of pain.

When our Mum was dying, in hospital, a nurse told us to talk to her as the hearing is usually the last sense to close down. We were there in shifts, sort of, so my brother and sister-in-law were there for her, but not me.

I was there for Dad, some years earlier, and he went through the common phase of what's called Cheyne-Stokes respiration - a cycle of stop-start breathing that tends to make you think the death has occurred before it actually does.  

So the normal, natural end is slow but peaceful, and the dying person is out of distress.